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/gitcmd/error_test.go

@@ -14,8 +14,6 @@ func TestIsStderr(t *testing.T) {
 		check  StderrCheck
 		stderr string
 	}{
-		{StderrUnknownRevisionOrPath, "fatal: ambiguous argument 'origin': unknown revision or path not in the working tree...."},
-		{StderrNoMergeBase, "fatal: origin/main..HEAD: no merge base...."},
 		{StderrFileNoEnoughLines, "fatal: file foo/bar has only 1 line"},
 		{StderrAuthenticationFailed, "Cloning into 'repo'...\r\nremote: Invalid username or token.\r\nfatal: Authentication failed for 'https://host/repo.git/'\r\n"},
 	}