Move expire_reflog() into refs.c and rename it to reflog_expire().
Turn the three policy functions into function pointers that are passed
into reflog_expire(). Add function prototypes and documentation to
refs.h.
[jc: squashed in $gmane/261582, drop "extern" in function definition]
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu> Reviewed-by: Stefan Beller <sbeller@google.com> Tweaked-by: Ramsay Jones Signed-off-by: Junio C Hamano <gitster@pobox.com>