rust/chg/build.rs

merge: short-circuit the _checkfs loop upon getting ENOENT

This reduces the number of [lstat] calls when updating from rev(-1) to
a rev with lots of files by a factor of several: for path foo/bar/baz/quux.txt
without this patch we're lstatting:

foo
foo/bar
foo/bar/baz
foo/bar/baz/quux.txt

and with this patch:

foo
foo/bar/baz/quux.txt

fn main() {
    cc::Build::new()
        .warnings(true)
        .file("src/sendfds.c")
        .file("src/sighandlers.c")
        .compile("procutil");
}