match: avoid rust fast path if the matcher was tampered with
Otherwise the fast path does not respect the modifications made
by the extension (concretely largefiles, but other extensions can
start using that too)
Set up
$ hg init repo
$ cd repo
$ echo a > a
$ hg add a
$ hg commit -m a
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