refactor(git): replace git stderr matching with exit codes (#39866)

Matching git's stderr text is fragile because it breaks silently when
git rewords a message. Where git offers an exit code or a query with a
defined answer, use that instead, and drop matches that could never
trigger. Follow-up to the discussion in https://github.com/go-gitea/gitea/pull/39857.

- Detect a missing remote by `git remote` exit code 2
- Resolve annotated tags to their tree with cat-file `^{tree}` instead
of spawning `ls-tree` when listing tree entries
- Skip the wiki page lookup when the default branch doesn't exist yet
- Remove unreachable "no merge base" and "unknown revision" checks

modules/git/repo_compare.go

@@ -46,11 +46,6 @@ func (repo *Repository) GetDiffNumChangedFiles(ctx context.Context, base, head s
 		WithRepo(repo).
 		WithStdoutCopy(w).
 		RunWithStderr(ctx); err != nil {
-		if gitcmd.IsStderr(err, gitcmd.StderrNoMergeBase) {
-			// git >= 2.28 now returns an error if base and head have become unrelated.
-			// it doesn't make sense to count the changed files in this case because UI won't display such diff
-			return 0, nil
-		}
 		return 0, err
 	}
 	return w.numLines, nil