Add ability to use accurev update instead of accurev pop

[orao]

It should be possible to specify using the accurev update -t <number> for use in a Workspace instead of the accurev pop -t <number> command.

If this was added as a configuration option then this script could be used to also convert old Accurev depots (i.e. Accurev 4.7 which doesn't support accurev pop -t <number>).

If you could add this I will point all users of ac2git that are on older versions of Accurev here.