fix(api): refactor branch and tag existence checks (#30618)

- Update branch existence check to also include tag existence check
- Adjust error message for branch/tag existence check

ref: go-gitea/gitea#30349

---------

Signed-off-by: appleboy <appleboy.tw@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>
(cherry picked from commit 6459c50278906893f3cbc2bf3e52eff65e739b37)

routers/api/v1/repo/pull.go

@@ -1079,11 +1079,10 @@ func parseCompareInfo(ctx *context.APIContext, form api.CreatePullRequestOption)
 	}
 
 	ctx.Repo.PullRequest.SameRepo = isSameRepo
-	log.Info("Base branch: %s", baseBranch)
-	log.Info("Repo path: %s", ctx.Repo.GitRepo.Path)
+	log.Trace("Repo path: %q, base branch: %q, head branch: %q", ctx.Repo.GitRepo.Path, baseBranch, headBranch)
 	// Check if base branch is valid.
-	if !ctx.Repo.GitRepo.IsBranchExist(baseBranch) {
-		ctx.NotFound("IsBranchExist")
+	if !ctx.Repo.GitRepo.IsBranchExist(baseBranch) && !ctx.Repo.GitRepo.IsTagExist(baseBranch) {
+		ctx.NotFound("BaseNotExist")
 		return nil, nil, nil, nil, "", ""
 	}
 
@@ -1146,7 +1145,7 @@ func parseCompareInfo(ctx *context.APIContext, form api.CreatePullRequestOption)
 	}
 
 	// Check if head branch is valid.
-	if !headGitRepo.IsBranchExist(headBranch) {
+	if !headGitRepo.IsBranchExist(headBranch) && !headGitRepo.IsTagExist(headBranch) {
 		headGitRepo.Close()
 		ctx.NotFound()
 		return nil, nil, nil, nil, "", ""