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
#if no-windows no-osx
$ mkdir -p xdgconf/hg
$ echo '[ui]' > xdgconf/hg/hgrc
$ echo 'username = foobar' >> xdgconf/hg/hgrc
$ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
$ unset HGRCPATH
$ hg config ui.username 2>/dev/null
foobar
#endif