for_each_reflog(): reimplement using iterators

Allow references with reflogs to be iterated over using a ref_iterator.
The latter is implemented as a files_reflog_iterator, which in turn uses
dir_iterator to read the "logs" directory.

Note that reflog iteration doesn't correctly handle per-worktree
reflogs (either before or after this patch).

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs/refs-internal.h

@@ -404,6 +404,13 @@ struct ref_iterator *files_ref_iterator_begin(const char *submodule,
 					      const char *prefix,
 					      unsigned int flags);
 
+/*
+ * Iterate over the references in the main ref_store that have a
+ * reflog. The paths within a directory are iterated over in arbitrary
+ * order.
+ */
+struct ref_iterator *files_reflog_iterator_begin(void);
+
 /* Internal implementation of reference iteration: */
 
 /*