expire_reflog(): extract two policy-related functions

expire_reflog(): extract two policy-related functions

Extract two functions, reflog_expiry_prepare() and
reflog_expiry_cleanup(), from expire_reflog(). This is a further step
towards separating the code for deciding on expiration policy from the
code that manages the physical deletion of reflog entries.

This change requires a couple of local variables from expire_reflog()
to be turned into fields of "struct expire_reflog_cb". More
reorganization of the callback data will follow in later commits.

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