An internal function used to implement "git checkout @{-1}" was
hard to use correctly.
* jc/reflog-reverse-walk:
refs.c: fix fread error handling
reflog: add for_each_reflog_ent_reverse() API
for_each_recent_reflog_ent(): simplify opening of a reflog file
for_each_reflog_ent(): extract a helper to process a single entry