git_config: Fix bugs in processing RepoConfig.

- Automatically create all the parent directories for `git confing
  --file ...`
- Prevent `RepoConfig` from inheriting caches from `GitConfig`. These
  two are different files.

Bug: 569927750
Test: ./run_tests
Change-Id: I9c60cecff75c1fb62be3ff63425d9618c0f56b06
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/643263
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Insaf Latypov <saferif@google.com>
Commit-Queue: Insaf Latypov <saferif@google.com>

git_config.py

@@ -266,6 +266,13 @@ class GitConfig:
         except KeyError:
             old = []
 
+        if self.file != self._SYSTEM_CONFIG:
+            # `git config --file` does not create missing parent directories
+            # (e.g. ~/.repoconfig/ on first use).
+            parent_dir = os.path.dirname(self.file)
+            if parent_dir:
+                os.makedirs(parent_dir, exist_ok=True)
+
         if value is None:
             if old:
                 del self._cache[key]
@@ -444,6 +451,11 @@ class GitConfig:
 class RepoConfig(GitConfig):
     """User settings for repo itself."""
 
+    # Keep separate caches from GitConfig, otherwise ForUser()/ForSystem()
+    # would return the already-cached ~/.gitconfig instance of the parent.
+    _ForUser = None
+    _ForSystem = None
+
     @staticmethod
     def _getUserConfig():
         repo_config_dir = os.getenv("REPO_CONFIG_DIR", os.path.expanduser("~"))