git_config: prefer XDG config location

Currently, repo ignores the XDG path for the git config file, and
creates a new one in the user's home directory. This commit changes the
behavior to prefer the XDG path if it exists, which matches git behavior
and avoids littering the home directory.

Bug: 40012443
Change-Id: Icd3ec6db6b0832f47417bbe98ff9461306b51297
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/448385


Tested-by: lmaor xenix <25misha52@gmail.com>
Reviewed-by: Mike Frysinger <vapier@google.com>

git_config.py

@@ -90,6 +90,20 @@ class GitConfig:
 
     @staticmethod
     def _getUserConfig():
+        """Get the user-specific config file.
+
+        Prefers the XDG config location if available, with fallback to
+        ~/.gitconfig
+
+        This matches git behavior:
+        https://git-scm.com/docs/git-config#FILES
+        """
+        xdg_config_home = os.getenv(
+            "XDG_CONFIG_HOME", os.path.expanduser("~/.config")
+        )
+        xdg_config_file = os.path.join(xdg_config_home, "git", "config")
+        if os.path.exists(xdg_config_file):
+            return xdg_config_file
         return os.path.expanduser("~/.gitconfig")
 
     @classmethod