Cache the maximum revision for each rev_db URL rather than looking it
up each time. This saves a lot of time when rebuilding indexes on a
freshly cloned repository.
Signed-off-by: Sam Vilain <sam.vilain@catalyst.net.nz> Acked-by: Eric Wong <normalperson@yhbt.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>