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

routers/web/repo/pull.go

@@ -370,8 +370,7 @@ func (prInfo *pullRequestViewInfo) prepareViewFillCompareInfo(ctx *context.Conte
 	pull := prInfo.issue.PullRequest
 	prInfo.CompareInfo, err = git_service.GetCompareInfo(ctx, ctx.Repo.Repository, ctx.Repo.Repository, ctx.Repo.GitRepo, baseRef, git.RefName(pull.GetGitHeadRefName()), false, false)
 	if err != nil {
-		isKnownErrorForBroken := errors.Is(err, util.ErrNotExist) || gitcmd.IsStderr(err, gitcmd.StderrNotValidObjectName) || gitcmd.IsStderr(err, gitcmd.StderrUnknownRevisionOrPath)
-		if !isKnownErrorForBroken {
+		if !errors.Is(err, util.ErrNotExist) && !gitcmd.IsStderr(err, gitcmd.StderrNotValidObjectName) {
 			log.Error("GetCompareInfo: %v", err)
 		}
 		prInfo.IsPullRequestBroken = true