[scm_mock] Make params keyword only

None of the optional params are used currently, so make them keyword
to ease future changes, like adding the global_config in the next CL.

Change-Id: I213cccb945c969b4211adad8ccd9170ef6209e0a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5773568
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Allen Li <ayatane@chromium.org>

tests/scm_mock.py

@@ -18,6 +18,7 @@ import scm
 
 
 def GIT(test: unittest.TestCase,
+        *,
         branchref: str | None = None):
     """Installs fakes/mocks for scm.GIT so that: