git: add active_branch_remote for looking up active tracking-branch

remote

src/scmrepo/git/backend/base.py

@@ -136,6 +136,10 @@ def is_dirty(self, untracked_files: bool = False) -> bool:
     def active_branch(self) -> str:
         pass
 
+    @abstractmethod
+    def active_branch_remote(self) -> str:
+        """Return the fetch remote name for the current branch."""
+
     @abstractmethod
     def list_branches(self) -> Iterable[str]:
         pass

src/scmrepo/git/backend/dulwich/__init__.py

@@ -461,6 +461,9 @@ def is_dirty(self, untracked_files: bool = False) -> bool:
     def active_branch(self) -> str:
         raise NotImplementedError
 
+    def active_branch_remote(self) -> str:
+        raise NotImplementedError
+
     def list_branches(self) -> Iterable[str]:
         base = "refs/heads/"
         return sorted(ref[len(base) :] for ref in self.iter_refs(base))

src/scmrepo/git/backend/gitpython.py

@@ -344,6 +344,12 @@ def active_branch(self):
         except TypeError as exc:
             raise SCMError("No active branch") from exc
 
+    def active_branch_remote(self) -> str:
+        try:
+            return self.repo.active_branch.tracking_branch()
+        except (TypeError, ValueError) as exc:
+            raise SCMError("No active branch tracking remote") from exc
+
     def list_branches(self):
         return [h.name for h in self.repo.heads]
 

src/scmrepo/git/backend/pygit2/__init__.py

@@ -389,6 +389,15 @@ def active_branch(self) -> str:
             return self.repo.references["HEAD"].target[11:]
         return self.repo.head.shorthand
 
+    def active_branch_remote(self) -> str:
+        try:
+            upstream = self.repo.branches[self.active_branch()].upstream
+            if upstream:
+                return upstream.remote_name
+        except (KeyError, ValueError):
+            pass
+        raise SCMError("No active branch tracking remote")
+
     def list_branches(self) -> Iterable[str]:
         base = "refs/heads/"
         return sorted(ref[len(base) :] for ref in self.iter_refs(base))