Add "--expire <time>" option to 'git prune'

Add "--expire <time>" option to 'git prune'

Earlier, 'git prune' would prune all loose unreachable objects.
This could be quite dangerous, as the objects could be used in
an ongoing operation.

This patch adds a mode to expire only loose, unreachable objects
which are older than a certain time.  For example, by

	git prune --expire 14.days

you can prune only those objects which are loose, unreachable
and older than 14 days (and thus probably outdated).

The implementation uses st.st_mtime rather than st.st_ctime,
because it can be tested better, using 'touch -d <time>' (and
omitting the test when the platform does not support that
command line switch).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>