feature: when merging environments, promote tag from previous env to next env

src/git_flow/remote/gitlab.py

@@ -49,14 +49,7 @@ class GitlabRemoteAPI(RemoteAPI):
             raise GitFlowError("Ocurrió un error al crear el tag: " + response.text)
 
     def get_merge_commits(self, merge_commit: str) -> list[Commit]:
-        mrs = self.get_json(
-            self.get_endpoint(f"/repository/commits/{merge_commit}/merge_requests")
-        ).json()
-        mr = next((m for m in mrs if m.get("merge_commit_sha") == merge_commit), None)
-
-        if not mr:
-            raise GitFlowError(f"No se encontró un MR para el commit {merge_commit}")
-
+        mr = self._get_merge_request(merge_commit)
         items = self.get_all_pages(self.get_endpoint(f"/merge_requests/{mr['iid']}/commits"), 100)
 
         # GitLab ya los devuelve del más nuevo al más viejo
@@ -70,6 +63,20 @@ class GitlabRemoteAPI(RemoteAPI):
             for c in items
         ]
 
+    def get_merge_source_branch(self, merge_commit: str) -> str:
+        return self._get_merge_request(merge_commit)["source_branch"]
+
+    def _get_merge_request(self, merge_commit: str) -> dict:
+        mrs = self.get_json(
+            self.get_endpoint(f"/repository/commits/{merge_commit}/merge_requests")
+        ).json()
+        mr = next((m for m in mrs if m.get("merge_commit_sha") == merge_commit), None)
+
+        if not mr:
+            raise GitFlowError(f"No se encontró un MR para el commit {merge_commit}")
+
+        return mr
+
     def list_tags(self, suffix: str = "") -> list[str]:
         # "search" soporta "$" para anclar al final del nombre
         params = {"search": suffix + "$"} if suffix else None