Fix switching branches by adding double dash separator (#1390)

* Add hyphens to treat params as Git objects and not files

* Adjust test assertions

Author: Paul2708

Diff for: jupyterlab_git/git.py

@@ -312,7 +312,7 @@ async def changed_files(self, path, base=None, remote=None, single_commit=None):
             elif base == "INDEX":
                 cmd = ["git", "diff", "--staged", remote, "--name-only", "-z"]
             else:
-                cmd = ["git", "diff", base, remote, "--name-only", "-z"]
+                cmd = ["git", "diff", base, remote, "--name-only", "-z", "--"]
         else:
             raise tornado.web.HTTPError(
                 400, "Either single_commit or (base and remote) must be provided"

Diff for: jupyterlab_git/tests/test_diff.py

@@ -112,7 +112,7 @@ async def test_changed_files_two_commits():
 
         # Then
         mock_execute.assert_called_once_with(
-            ["git", "diff", "HEAD", "origin/HEAD", "--name-only", "-z"],
+            ["git", "diff", "HEAD", "origin/HEAD", "--name-only", "-z", "--"],
             cwd="test-path",
             timeout=20,
             env=None,
@@ -136,7 +136,7 @@ async def test_changed_files_git_diff_error():
 
         # Then
         mock_execute.assert_called_once_with(
-            ["git", "diff", "HEAD", "origin/HEAD", "--name-only", "-z"],
+            ["git", "diff", "HEAD", "origin/HEAD", "--name-only", "-z", "--"],
             cwd="test-path",
             timeout=20,
             env=None,