merge: avoid dereferencing repo fields repeatedly

merge: skip syntactic path checks in [_checkunknownfile] We don't need to check the paths syntactically, since they are coming from diffing the revisions, so hopefully already checked on the way in. We still need to check what's on the filesystem, to avoid traversing the symlinks or subdirs, which we can't know about statically. Also, we use the directory audit to elide [isfileorlink], this removing ~all lstat calls from hg updates from-empty.

pathutil: use `finddirs_rev_noroot` instead of `parts` The benefit this brings is very tiny, if it's even there, since we still didn't get rid of the [parts] computation. It probably won't be worth it without the subsequent patch that adds one more use of [finddirs_rev_noroot]