bundles: do not overwrite existing backup bundles (BC) Previously, a backup bundle could overwrite an existing bundle and cause user data loss. For instance, if you have A<-B<-C and strip B, it produces backup bundle B-backup.hg. If you then hg pull -r B B-backup.hg and strip it again, it overwrites the existing B-backup.hg and C is lost. The fix is to add a hash of all the nodes inside that bundle to the filename. Fixed up existing tests and added a new test in test-strip.t