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
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
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$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing::
$ make local # build for inplace usage
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See https://mercurial-scm.org/ for detailed installation
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