forge--set-topic-labels: Properly encode old label IDs

I really messed up ID handling in the early days.  Fixing that will
force all user has to recreate their local database from scratch,
which is why I still haven't done it.

Anyway, this is becoming more of a problem now that we are switching
to using GraphQL even for mutations.  While the REST API usually takes
numbers and names to identify objects, the GraphQL API expects IDs.

`base64-decode-string' on our label IDsgives us a string using one of
the following two forms.  (These are the "support" and "area: github"
labels of this repository.)

(base64-decode-string "Z2l0aHViLmNvbTowMTA6UmVwb3NpdG9yeTE0MTM3ODE5NDowNTpMYWJlbDExODMxMDE5ODg=")
=> "github.com:010:Repository141378194:05:Label1183101988"

(base64-decode-string "Z2l0aHViLmNvbTowMTA6UmVwb3NpdG9yeTE0MTM3ODE5NDpMQV9rd0RPQ0cxQ2tzOEFBQUFCY0hhQzdR")
=> "github.com:010:Repository141378194:LA_kwDOCG1Cks8AAAABcHaC7Q"

In the latter case we can use "LA_kwDOCG1Cks8AAAABcHaC7Q" as-is, in the
former case we have to encode "05:Label1183101988" to get the ID used on
Github, "MDU6TGFiZWwxMTgzMTAxOTg4".

(base64-encode-string "05:Label1183101988")
=> "MDU6TGFiZWwxMTgzMTAxOTg4"

It seemed like a good idea, in the "we might as well" category, to try
to decode the upstream ID and, if that succeeded, combine that (else
the original ID as-is) with our package prefix, before encoding the
combination, to avoid "double-encoding".  This broke down once Github
changed the format of their IDs and we now have to deal with both their
new and legacy formats.

Closes #784.

Author: tarsius

CHANGELOG

@@ -1,4 +1,10 @@
 # -*- mode: org -*-
+* v0.5.3    UNRELEASED
+
+- Fixed a regression in v0.5.1, which prevented labeling topics
+  with labels that were created before Github changed the format
+  of label IDs.  #784
+
 * v0.5.2    2025-06-03
 
 - Fixed a regression in v0.5.1, which prevented the creation of a

lisp/forge-github.el

@@ -955,8 +955,15 @@
 (cl-defmethod forge--set-topic-labels
   ((repo forge-github-repository) topic labels)
   (let* ((topic-id (oref topic their-id))
-         (old (mapcar (##forge--their-id (car %)) (oref topic labels)))
-         (new (mapcar (##forge--their-id (car %))
+         (their-id ; I really messed up IDs! :(
+          (pcase-lambda (`(,id))
+            (let* ((parts (split-string (base64-decode-string id) ":"))
+                   (maybe-id (car (last parts))))
+              (if (string-prefix-p "Label" maybe-id)
+                  (base64-encode-string (string-join (last parts 2) ":"))
+                maybe-id))))
+         (old (mapcar their-id (oref topic labels)))
+         (new (mapcar their-id
                       (forge-sql [:select [id] :from label
                                   :where (and (= repository $s1)
                                               (in name $v2))]