rhg: add support for ignoring all extensions
Some workflows just want what `rhg` does and don't care about any extensions,
this makes it easier.
Differential Revision: https://phab.mercurial-scm.org/D12385
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
[10]
No dirstate backups are left behind
$ ls .hg/dirstate* | sort
.hg/dirstate