revlog: avoid opening and closing the file for each cloned revision
The previous code was flushing files after each new revision, slowing things
down.
For exemple, with this change, the evolve repository can run
`hg debugupgraderepo --run --optimize re-delta-parent` in about 3.4s instead of
4.5 seconds.
# __init__.py - Startup and module loading logic for Mercurial.
#
# Copyright 2015 Gregory Szorc <gregory.szorc@gmail.com>
#
# This software may be used and distributed according to the terms of the
# GNU General Public License version 2 or any later version.
# Allow 'from mercurial import demandimport' to keep working.
import hgdemandimport
demandimport = hgdemandimport