fix: incorrect co-author detection on commit page (#38386)

The commit page built its "co-authored by" list from
`AllParticipantIdentities()[1:]`, which only drops the author and leaves
the committer in the list even though the committer is already shown
separately as "committed by". This caused the committer to be wrongly
rendered as a co-author (issue #38384): a commit authored by
`silverwind`, committed by `bircni`, with a `Co-authored-by: silverwind`
trailer displayed "co-authored by bircni" instead of the actual trailer
identity. This adds a `Commit.CoAuthorIdentities()` method that excludes
both the author and the committer, uses it on the commit page, and
covers the fix with a regression test.

Closes #38384

routers/web/repo/commit.go

@@ -398,7 +398,7 @@ func Diff(ctx *context.Context) {
 	verification := asymkey_service.ParseCommitWithSignature(ctx, commit)
 	ctx.Data["Verification"] = verification
 	ctx.Data["Author"] = user_model.GetUserByGitAuthor(ctx, commit)
-	ctx.Data["CommitOtherParticipants"] = gituser.BuildAvatarStackData(ctx, commit.AllParticipantIdentities(), nil).Participants[1:]
+	ctx.Data["CommitOtherParticipants"] = gituser.BuildAvatarStackData(ctx, commit.CoAuthorIdentities(), nil).Participants
 	ctx.Data["Parents"] = parents
 	ctx.Data["DiffNotAvailable"] = diffShortStat.NumFiles == 0