copies: avoid materializing a full directory map during copy tracing
Materializing a full copy of every directory in a treemanifest repo can be quite
expensive, even with a narrow matcher. For flat manifest repos, this should be
equivalent - it will still materialize (and cache) a dict of all of the dirs
inside of the manifest object, we just don't get a copy of it.
In a repo I have here, this brings the time for a simple rebase from 11.197s to
4.609s.
Differential Revision: https://phab.mercurial-scm.org/D9503
$ hg init repo
$ cd repo
$ touch foo
$ hg ci -Am 'add foo'
adding foo
$ hg up -C null
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
this should be stored as a delta against rev 0
$ echo foo bar baz > foo
$ hg ci -Am 'add foo again'
adding foo
created new head
$ hg debugindex foo
rev linkrev nodeid p1 p2
0 0 b80de5d13875 000000000000 000000000000
1 1 0376abec49b8 000000000000 000000000000
$ cd ..