fix: parse change-id in the git commit header (#7884)

- `change-id` is a commit header set by [jj-vcs](https://jj-vcs.github.io/jj/). Modify the commit parser to consider this header to be a valid header, this in turn fixes the signature validation on git commits that contain this header.
- Resolves forgejo/forgejo#7836
- Added unit test.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7884
Reviewed-by: Otto <otto@codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

modules/git/commit_reader.go

@@ -85,6 +85,8 @@ readLoop:
 				_, _ = payloadSB.Write(line)
 			case "encoding":
 				_, _ = payloadSB.Write(line)
+			case "change-id": // jj-vcs specific header.
+				_, _ = payloadSB.Write(line)
 			case "gpgsig":
 				fallthrough
 			case "gpgsig-sha256": // FIXME: no intertop, so only 1 exists at present.