gc.sh: compact non-current reflogs

gc.sh: compact non-current reflogs

If there's a large amount of activity on a project and reflogs are kept
for many days, they could potentially consume a fair amount of space
(although still likely much less than the repository itself).

Mitigate this by gzip compressing log files for other than the current
date to greatly reduce the space required while being very careful to
handle "accidents" where both a compressed and uncompressed log file for
the same date end up somehow coexisting simultaneously.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>