scmutil: speed up revrange
This improves the performance of "hg log -l1" from 0.21 seconds to
0.07 on a Linux kernel tree.
Ideally we could use xrange instead of range on the most common
path, and thus avoid a ton of allocation, but xrange doesn't support
slice-based indexing.
graft: add --dry-run support (
issue3362)
repair: allow giving strip backup a different name
So the user can differentiate amend backups from the rest.