Fix gitea_api.Git.get_branch_head() to raise a proper exception if the HEAD cannot be retrieved

Author: dmach

osc/gitea_api/git.py

@@ -156,7 +156,10 @@ def get_branch_head(self, branch: Optional[str] = None) -> str:
         if not branch:
             branch = self.current_branch
 
-        return self._run_git(["rev-parse", f"refs/heads/{branch}"])
+        try:
+            return self._run_git(["rev-parse", f"refs/heads/{branch}"], mute_stderr=True)
+        except subprocess.CalledProcessError:
+            raise exceptions.GitObsRuntimeError(f"Unable to retrieve HEAD from branch '{branch}'. Does the branch exist?")
 
     def branch_exists(self, branch: str) -> bool:
         try: