config: clarify memory ownership in `git_config_string()`

The out parameter of `git_config_string()` is a `const char **` even
though we transfer ownership of memory to the caller. This is quite
misleading and has led to many memory leaks all over the place. Adapt
the parameter to instead be `char **`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/commit.c b/builtin/commit.c
index 1cc88e9..f53e7e8 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -133,7 +133,7 @@ static struct strvec trailer_args = STRVEC_INIT;
  * is specified explicitly.
  */
 static enum commit_msg_cleanup_mode cleanup_mode;
-static const char *cleanup_arg;
+static char *cleanup_arg;
 
 static enum commit_whence whence;
 static int use_editor = 1, include_status = 1;