fix(api): handle missing base branch in PR commits API (#37193) (#37203)

Backport #37193 by @Mohit25022005

fix(api): handle missing base branch in PR commits API

Closes #36366

Previously, the PR commits API returned a 500 Internal Server Error
when the base branch was missing due to an unhandled git "bad revision"
error.

This change:
- Checks for base branch existence before performing git operations
- Returns 404 when the base branch does not exist
- Prevents git errors from surfacing as 500 responses

This improves API robustness and aligns behavior with expected error
handling.

Tested locally by:
- Creating a pull request
- Deleting the base branch
- Verifying that the API returns 404 instead of 500

Co-authored-by: Mohit Swarnkar <mohitswarnkar13@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

routers/api/v1/repo/pull.go

@@ -22,6 +22,7 @@ import (
 	user_model "code.gitea.io/gitea/models/user"
 	"code.gitea.io/gitea/modules/base"
 	"code.gitea.io/gitea/modules/git"
+	"code.gitea.io/gitea/modules/git/gitcmd"
 	"code.gitea.io/gitea/modules/gitrepo"
 	"code.gitea.io/gitea/modules/graceful"
 	"code.gitea.io/gitea/modules/log"
@@ -1429,7 +1430,11 @@ func GetPullRequestCommits(ctx *context.APIContext) {
 	} else {
 		compareInfo, err = git_service.GetCompareInfo(ctx, pr.BaseRepo, pr.BaseRepo, baseGitRepo, git.RefNameFromBranch(pr.BaseBranch), git.RefName(pr.GetGitHeadRefName()), false, false)
 	}
-	if err != nil {
+
+	if gitcmd.StderrHasPrefix(err, "fatal: bad revision") {
+		ctx.APIError(http.StatusNotFound, "invalid base branch or revision")
+		return
+	} else if err != nil {
 		ctx.APIErrorInternal(err)
 		return
 	}