rust: add UnionMatcher
This will be used in the upcoming support for sparse checkouts in
Rust-augmented status and later in rhg support for sparse checkouts.
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