fsck.c: refactor and rename common config callback

Refactor code I recently changed in 1f3299fda9 (fsck: make
fsck_config() re-usable, 2021-01-05) so that I could use fsck's config
callback in mktag in 1f3299fda9 (fsck: make fsck_config() re-usable,
2021-01-05).

I don't know what I was thinking in structuring the code this way, but
it clearly makes no sense to have an fsck_config_internal() at all
just so it can get a fsck_options when git_config() already supports
passing along some void* data.

Let's just make use of that instead, which gets us rid of the two
wrapper functions, and brings fsck's common config callback in line
with other such reusable config callbacks.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/fsck.c b/fsck.c
index e3030f3..5dfb996 100644
--- a/fsck.c
+++ b/fsck.c
@@ -1323,9 +1323,9 @@ int fsck_finish(struct fsck_options *options)
 	return ret;
 }
 
-int fsck_config_internal(const char *var, const char *value, void *cb,
-			 struct fsck_options *options)
+int git_fsck_config(const char *var, const char *value, void *cb)
 {
+	struct fsck_options *options = cb;
 	if (strcmp(var, "fsck.skiplist") == 0) {
 		const char *path;
 		struct strbuf sb = STRBUF_INIT;