This vendors backups extension from hg-experimental.
Listing backups and having some utility to apply them is nice. I know we have
obsmarkers now, but this will help a lot of end users who still uses strip until
we get evolve out of experimental.
This is RFC because:
* I am not sure whether we want this in core
* I copied the code from hg-experimental and looks like there is some code
deduplication which needs to be done