Per-ref reflog expiry configuration

Per-ref reflog expiry configuration

In addition to gc.reflogexpireunreachable and gc.reflogexpire, this lets
you set gc.<pattern>.reflogexpireunreachable and gc.<pattern>.reflogexpire
variables.

When "git reflog expire" expires reflog entry for $ref, the expiry timers
are taken from the first <pattern> that matches $ref (and if there isn't
the global default value is used).

For example, you could:

	[gc "refs/stash"]
		reflogexpire = never
		reflogexpireunreachable = never

	[gc "refs/remotes/*"]
		reflogexpire = 7 days
		reflogexpireunreachable = 3 days

	[gc]
		reflogexpire = 90 days
		reflogexpireunreachable = 30 days

Signed-off-by: Junio C Hamano <gitster@pobox.com>