fix(git): interpret bare HEAD through the shared resolver and stop overwriting .Upstream with the remote list

[e-kulikov]

Fixes #7799.

Prerequisites

• I have read and understood the contributing guide.
• The commit message follows the conventional commits guidelines.
• Tests for the changes have been added.
• Docs have been added/updated where relevant.

Description

getBareRepoInfo() hand-rolled a HEAD parse that only understood ref: refs/heads/… — a reftables
repository rendered .Ref as .invalid, and a detached bare HEAD rendered the full 40-character
object ID with .Detached false and .Hash/.ShortHash empty, even though the non-bare path already
handles all of this correctly. Separately, .Upstream was assigned the raw, newline-separated output
of git remote — with two remotes configured, that put a literal line break in the middle of the
prompt.

This PR routes bare HEAD interpretation through the same resolver the non-bare path already uses, and
replaces the .Upstream assignment with a real upstream/remote-selection lookup, resolving to the
branch's actual upstream (or empty, when none is configured) instead of the remote list. Most of the
directory-selection machinery this originally needed is no longer necessary, since #7798 already makes
the current checkout's git directory correct in every layout this PR touches.

Verified with new tests covering reftables, detached HEAD, single- and multi-remote bare repositories,
and the existing non-bare behaviour, confirmed unchanged.

This is part of a stacked series

next (includes #7801)
 └─ fix/git-worktree-common-dir                     → #7825
     └─ fix/git-bare-head-upstream    (this PR)     → #7799
         └─ feat/statusline-payload-pwd              → #7800

This branch adds one commit on top of fix/git-worktree-common-dir (#7825) and depends on it directly —
this PR's simplified HEAD resolution assumes that PR's directory-role fix has already landed. Until
#7825 merges, this PR's diff includes its commit too; it will then shrink to this PR's own commit.

[github-actions]

📦 Release binary size report

Compares this PR's release-equivalent build against the latest published release, per OS (amd64).

OS	Baseline	This PR	Delta
darwin	13.79 MB	13.79 MB	-0.0 KB (+0%)
linux	13.46 MB	13.46 MB	+0.0 KB (+0%)
windows	14.02 MB	13.98 MB	-38.3 KB (-0.27%)

🎉 Binary size shrank on at least one platform.

[JanDeDobbeleer]

Same with this one, we can reopen.

[e-kulikov]

Recreated as #7830, based on main with the branch rebased and my work squashed into a single commit, as requested.

This PR closed automatically when next was deleted, and gh pr reopen can't restore it because the base branch no longer exists. Since next had already landed in main, the replacement targets main directly rather than waiting for next to come back.

Same code, no functional change: the commit's patch-id is unchanged. It still stacks on the directory-role fix, now #7828.