config: expose repo_config_clear()

While we already have `repo_config_clear()` as an alternative to
`git_config_clear()` that doesn't rely on `the_repository`, it is not
exposed to callers outside of the config subsystem. Do so.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.h

@@ -651,6 +651,7 @@ int git_config_get_string_multi(const char *key,
  * Resets and invalidates the config cache.
  */
 void git_config_clear(void);
+void repo_config_clear(struct repository *repo);
 
 /**
  * Allocates and copies the retrieved string into the `dest` parameter for