Add backend support for branching from an arbitrary branch

Author: kgryte

jupyterlab_git/git.py

@@ -606,12 +606,13 @@ def reset_to_commit(self, commit_id, top_repo_path):
         )
         return my_output
 
-    def checkout_new_branch(self, branchname, current_path):
+    def checkout_new_branch(self, branchname, startpoint, current_path):
         """
         Execute git checkout <make-branch> command & return the result.
         """
+        cmd = ["git", "checkout", "-b", branchname, startpoint]
         p = Popen(
-            ["git", "checkout", "-b", branchname],
+            cmd,
             stdout=PIPE,
             stderr=PIPE,
             cwd=os.path.join(self.root_dir, current_path),

jupyterlab_git/handlers.py

@@ -320,7 +320,7 @@ def post(self):
         if data["checkout_branch"]:
             if data["new_check"]:
                 my_output = self.git.checkout_new_branch(
-                    data["branchname"], top_repo_path
+                    data["branchname"], data["startpoint"], top_repo_path
                 )
             else:
                 my_output = self.git.checkout_branch(data["branchname"], top_repo_path)