typing: add trivial type hints to `mercurial.scmutil`
There's still a lot to go, but there's a lot here already, so I tried to keep it
to obvious/trivial things. I didn't bother with contexts, matchers, and
revisions that can be `bytes | int | None`.
$ hg init
$ echo 123 > a
$ hg add a
$ hg commit -m "first" a
$ mkdir sub
$ echo 321 > sub/b
$ hg add sub/b
$ hg commit -m "second" sub/b
$ cat sub/b
321
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ cat sub/b 2>/dev/null || echo "sub/b not present"
sub/b not present
$ test -d sub || echo "sub not present"
sub not present