merge: use manifestdict.walk() instead of manifestdict.matches()
As with other patches in this series, this avoids making a
potentially-expensive copy of a manifest.
Differential Revision: https://phab.mercurial-scm.org/D8084
manifest: rewrite filesnotin to not make superfluous manifest copies
This also skips using diff() when all we care about is the filenames. I'm
expecting the built in set logic to be plenty fast. For really large manifests
with a matcher in play this should copy substantially less data around.
Differential Revision: https://phab.mercurial-scm.org/D8082