for_each_reflog(): reimplement using iterators

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 <redacted>
Signed-off-by: Michael Haggerty <redacted>
Signed-off-by: Junio C Hamano <redacted>