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
basic test for hg debugrebuildstate
$ hg init repo
$ cd repo
$ touch foo bar
$ hg ci -Am 'add foo bar'
adding bar
adding foo
$ touch baz
$ hg add baz
$ hg rm bar
$ hg debugrebuildstate
state dump after
$ hg debugstate --nodates | sort
n 644 -1 bar
n 644 -1 foo
status
$ hg st -A
! bar
? baz
C foo
$ cd ..