support gitconfig from env

Add exceptions to the git env so externally configured gitconfig
values set via GIT_CONFIG_KEY_<n>, GIT_CONFIG_VALUE_<n> and
GIT_CONFIG_COUNT get passed through.

pre_commit/git.py

@@ -41,9 +41,10 @@ def no_git_env(
     return {
         k: v for k, v in _env.items()
         if not k.startswith('GIT_') or
+        k.startswith(('GIT_CONFIG_KEY_', 'GIT_CONFIG_VALUE_')) or
         k in {
             'GIT_EXEC_PATH', 'GIT_SSH', 'GIT_SSH_COMMAND', 'GIT_SSL_CAINFO',
-            'GIT_SSL_NO_VERIFY',
+            'GIT_SSL_NO_VERIFY', 'GIT_CONFIG_COUNT',
         }
     }