config: teach repo_config to allow repo to be NULL

The `repo` value can be NULL if a builtin command is run outside
any repository. The current implementation of `repo_config()` will
fail if `repo` is NULL.

If the `repo` is NULL the `repo_config()` can ignore the repository
configuration but it should read the other configuration sources like
the system-side configuration instead of failing.

Teach the `repo_config()` to allow `repo` to be NULL by calling the
`read_very_early_config()` which read config but only enumerate system
and global settings.

This will be useful in the following commits.

Suggested-by: Junio C Hamano <[email protected]>
Mentored-by: Christian Couder <[email protected]>
Signed-off-by: Usman Akinyemi <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Unique-Usman

config.c

@@ -2521,6 +2521,10 @@ void repo_config_clear(struct repository *repo)
 
 void repo_config(struct repository *repo, config_fn_t fn, void *data)
 {
+	if (!repo) {
+		read_very_early_config(fn, data);
+		return;
+	}
 	git_config_check_init(repo);
 	configset_iter(repo->config, fn, data);
 }

config.h

@@ -219,6 +219,9 @@ void read_very_early_config(config_fn_t cb, void *data);
  * repo-specific one; by overwriting, the higher-priority repo-specific
  * value is left at the end).
  *
+ * In cases where the repository variable is NULL, repo_config() will
+ * call read_early_config().
+ *
  * Unlike git_config_from_file(), this function respects includes.
  */
 void repo_config(struct repository *r, config_fn_t fn, void *);