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/remote.go

@@ -64,9 +64,9 @@ func (err *ErrInvalidCloneAddr) Unwrap() error {
 	return util.ErrInvalidArgument
 }
 
-// IsRemoteNotExistError checks the prefix of the error message to see whether a remote does not exist.
+// IsRemoteNotExistError checks whether a "git remote" subcommand failed because the remote does not exist
 func IsRemoteNotExistError(err error) bool {
-	return gitcmd.IsStderr(err, gitcmd.StderrNoSuchRemote)
+	return gitcmd.IsErrorExitCode(err, 2)
 }
 
 // ParseRemoteAddr checks if given remote address is valid,