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