subtree: add --git-shallow-clone flag to subtree import

Summary:
Add a --git-shallow-clone flag to `sl subtree import` that fetches only the
requested commit using `git fetch --depth=1`. This is faster for importing
from large repositories since it avoids downloading full history.

The shallow-cloned Git repo is placed in a temp directory and deleted after
import, because a shallow clone lacks the commit history needed for blame
and log operations.

## Performance

### 1. facebook/sapling

Before
```
$ time sl subtree import --url https://github.com/facebook/sapling -r 5401649 --to-path fb_sl01 -m "import fb sl"
...
Receiving objects: 100% (1019726/1019726), 268.11 MiB | 48.80 MiB/s, done.
Resolving deltas: 100% (694194/694194), done.
...
________________________________________________________
Executed in   30.64 secs    fish           external
   usr time   42.47 secs    0.00 micros   42.47 secs
   sys time   17.92 secs  445.00 micros   17.92 secs
```

After
```
$ time sl subtree import --url https://github.com/facebook/sapling -r 5401649 --to-path fb_sl02 -m "import fb sl" --git-shallow-clone
...
Receiving objects: 100% (12613/12613), 29.62 MiB | 19.82 MiB/s, done.
Resolving deltas: 100% (2705/2705), done.
...
________________________________________________________
Executed in   17.35 secs    fish           external
   usr time    6.73 secs  408.00 micros    6.73 secs
   sys time    2.96 secs   27.00 micros    2.96 secs
```

### 2. python/cpython

Before
```
$ time sl subtree import --url https://github.com/python/cpython.git -r 36f15ba5cd15607fb4e4908ddbfb462c44626d6b --to-path cpython -m "import cpython"
...
________________________________________________________
Executed in   35.94 secs    fish           external
   usr time   81.70 secs  405.00 micros   81.70 secs
   sys time   13.57 secs   44.00 micros   13.57 secs
```

After
```
$ time sl subtree import --url https://github.com/python/cpython.git -r 36f15ba5cd15607fb4e4908ddbfb462c44626d6b --to-path cpython2 -m "import cpython" --git-shallow-clone
________________________________________________________
Executed in   15.44 secs    fish           external
   usr time    5.16 secs   40.71 millis    5.11 secs
   sys time    1.77 secs    4.97 millis    1.77 secs
```

Reviewed By: janezhang10

Differential Revision: D99630689

fbshipit-source-id: 5b912cd90884dd9b581d1f2ea4b41b189d107d5e

Author: zzl0

eden/scm/sapling/commands/subtree.py

@@ -3,8 +3,11 @@
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2.
 
+import contextlib
 import json
 import os
+import shutil
+import tempfile
 from collections import defaultdict
 from typing import List
 
@@ -158,6 +161,12 @@ def subtree_copy(ui, repo, *args, **opts):
             _("REV"),
         ),
         ("f", "force", None, _("overwrite existing path")),
+        (
+            "",
+            "git-shallow-clone",
+            None,
+            _("use git shallow clone (--depth=1) (EXPERIMENTAL)"),
+        ),
     ]
     + subtree_path_opts
     + commitopts
@@ -177,6 +186,11 @@ def subtree_import(ui, repo, *args, **opts):
     Optionally, a source path within the external repository can be specified to
     import only a subdirectory.
 
+    Use ``--git-shallow-clone`` to fetch only the requested commit without
+    full history (``git fetch --depth=1``). This is faster for large
+    repositories but future blame or log on the imported path may be slower
+    because they need to download the commit history.
+
     Examples:
 
     - Import entire repository::
@@ -825,6 +839,26 @@ def commitfunc(ui, repo, message, match, opts):
     cmdutil.commit(ui, repo, commitfunc, [], opts)
 
 
+@contextlib.contextmanager
+def _shallow_clone_git_repo(ui, url, from_rev):
+    """Clone a git repo with --depth=1 into a temp directory.
+
+    The cloned repo is not cached and is deleted after the context exits,
+    because a shallow clone does not support blame or log operations.
+    """
+    from .. import git
+
+    git_repo_dir = tempfile.mkdtemp(prefix="sl-subtree-")
+    # disable partial clone when shallow clone is enabled
+    overrides = {("git", "depth"): 1, ("git", "filter"): None}
+    ui.status(_("creating git repo at %s\n") % git_repo_dir)
+    try:
+        with ui.configoverride(overrides, "subtree-import"):
+            yield git.clone(ui, url, git_repo_dir, update=from_rev)
+    finally:
+        shutil.rmtree(git_repo_dir, ignore_errors=True)
+
+
 def _do_import(ui, repo, *args, **opts):
     cmdutil.bailifchanged(repo)
 
@@ -850,13 +884,18 @@ def _do_import(ui, repo, *args, **opts):
 
     abort_or_remove_paths(ui, repo, to_paths, "import", opts)
 
-    git_repo = get_or_clone_git_repo(ui, giturl, from_rev)
-    from_ctx = git_repo[from_rev]
-    subtreeutil.validate_path_exist(ui, from_ctx, from_paths, abort_on_missing=True)
-
-    copy_files(ui, git_repo, repo, from_ctx, from_paths, to_paths, "import")
+    if opts.get("git_shallow_clone"):
+        repo_ctx_manager = _shallow_clone_git_repo(ui, giturl, from_rev)
+    else:
+        repo_ctx_manager = contextlib.nullcontext(
+            get_or_clone_git_repo(ui, giturl, from_rev)
+        )
 
-    from_commit = from_ctx.hex()
+    with repo_ctx_manager as git_repo:
+        from_ctx = git_repo[from_rev]
+        from_commit = from_ctx.hex()
+        subtreeutil.validate_path_exist(ui, from_ctx, from_paths, abort_on_missing=True)
+        copy_files(ui, git_repo, repo, from_ctx, from_paths, to_paths, "import")
     # use the original `url` in the metadata, as the `giturl` may lost information
     # e.g.: "git+" prefix
     extra = subtreeutil.gen_import_info(ui, url, from_commit, from_paths, to_paths)

eden/scm/sapling/git.py

@@ -765,6 +765,9 @@ def pullrefspecs(repo, url, refspecs):
         # Nothing to pull
         return 0
     args = ["fetch", "--no-tags", "--prune"]
+    depth = repo.ui.configint("git", "depth")
+    if depth:
+        args.append(f"--depth={depth}")
     if repo.ui.configbool("git", "shallow"):
         filter_config = repo.ui.config("git", "filter")
         if filter_config:

eden/scm/tests/test-subtree-import-blame.t

@@ -60,3 +60,10 @@ Test blame support phabricator diff number
            : 1
    D1234567: 2
            : 3
+
+Test subtree import with --git-shallow-clone, blame should work
+
+  $ sl subtree import -q --url $GIT_URL --rev main --to-path bar2 -m "import gitrepo to bar2" --git-shallow-clone
+  $ sl blame bar2/alpha
+  b6c31add3e60~: 1
+  6a5b13188f04~: 2

eden/scm/tests/test-subtree-import-log.t

@@ -201,3 +201,24 @@ Test log.follow-xrepo config
   │  user:        test
   ~  date:        Thu Jan 01 00:00:00 1970 +0000
      summary:     import gitrepo to bar
+
+Test subtree import with --git-shallow-clone, log should work
+
+  $ sl subtree import -q --url $GIT_URL --rev main --to-path bar2 -m "import gitrepo to bar2" --git-shallow-clone
+  $ sl log bar2/alpha
+  commit:      * (glob)
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     import gitrepo to bar2
+  
+  commit:      6a5b13188f04~
+  bookmark:    remote/main
+  hoistedname: main
+  user:        test <test@example.org>
+  date:        Mon Jan 01 00:00:10 2007 +0000
+  summary:     update alpha\nhttps://phabricator.test.com/D1234567
+  
+  commit:      b6c31add3e60~
+  user:        test <test@example.org>
+  date:        Mon Jan 01 00:00:10 2007 +0000
+  summary:     alpha