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

services/mirror/mirror_pull.go

@@ -43,7 +43,7 @@ func UpdateAddress(ctx context.Context, m *repo_model.Mirror, addr string) error
 	}
 
 	err = git.ManagedRemoteAdd(ctx, repo, remoteName, addr, git.RemoteOptionMirrorFetch)
-	if err != nil && !git.IsRemoteNotExistError(err) {
+	if err != nil {
 		return err
 	}
 
@@ -56,7 +56,7 @@ func UpdateAddress(ctx context.Context, m *repo_model.Mirror, addr string) error
 		}
 
 		err = git.ManagedRemoteAdd(ctx, repo.WikiStorageRepo(), remoteName, wikiRemotePath, git.RemoteOptionMirrorFetch)
-		if err != nil && !git.IsRemoteNotExistError(err) {
+		if err != nil {
 			return err
 		}
 	}