revlog: catch revlog corruption in index_baserev
A revision cannot use a base above itself, it can only happens one corrupted
repository.
Ignoring such corrupted could lead to infinite loop.
Set up
$ hg init repo
$ cd repo
Try to import an empty patch
$ hg import --no-commit - <<EOF
> EOF
applying patch from stdin
abort: stdin: no diffs found
[255]
No dirstate backups are left behind
$ ls .hg/dirstate* | sort
.hg/dirstate