According to profile data, _rev_list and rebuild consume a large
portion of time. Memoize the results of _rev_list and memoize
rebuild internals to avoid subprocess invocation.
When importing 15152 revisions on a LAN, time improved from 10
hours to 3-4 hours.
Signed-off-by: lin zuojian <manjian2006@gmail.com> Signed-off-by: Eric Wong <normalperson@yhbt.net>