[scm_mock] Expose mocking of system config layer.

R=ayatane, yiwzhang

Change-Id: I15a48c6af54d2b047b085abc729d2849759816d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5762792
Reviewed-by: Allen Li <ayatane@chromium.org>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Yiwei Zhang <yiwzhang@google.com>

tests/scm_mock.py

@@ -18,9 +18,12 @@ sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
 import scm
 
 
-def GIT(test: unittest.TestCase,
+def GIT(
+    test: unittest.TestCase,
     *,
-        branchref: str | None = None) -> Iterable[tuple[str, list[str]]]:
+    branchref: str | None = None,
+    system_config: dict[str, list[str]] | None = None
+) -> Iterable[tuple[str, list[str]]]:
     """Installs fakes/mocks for scm.GIT so that:
 
       * GetBranch will just return a fake branchname starting with the value of
@@ -28,12 +31,13 @@ def GIT(test: unittest.TestCase,
       * git_new_branch.create_new_branch will be mocked to update the value
         returned by GetBranch.
 
+    If provided, `system_config` allows you to set the 'system' scoped
+    git-config which will be visible as the immutable base configuration layer
+    for all git config scopes.
+
     NOTE: The dependency on git_new_branch.create_new_branch seems pretty
     circular - this functionality should probably move to scm.GIT?
     """
-    # TODO - add `system_config` - this will be configuration which exists at
-    # the 'system installation' level and is immutable.
-
     _branchref = [branchref or 'refs/heads/main']
 
     global_lock = threading.Lock()
@@ -45,7 +49,7 @@ def GIT(test: unittest.TestCase,
     patches: list[mock._patch] = [
         mock.patch('scm.GIT._new_config_state',
                    side_effect=lambda _: scm.GitConfigStateTest(
-                       global_lock, global_state)),
+                       global_lock, global_state, system_state=system_config)),
         mock.patch('scm.GIT.GetBranchRef', side_effect=lambda _: _branchref[0]),
         mock.patch('git_new_branch.create_new_branch', side_effect=_newBranch)
     ]