revlog: unify flag processing when loading index
The new code use a simple declaration to do centralised processing. This is
clearer, shorter and less error prone. This will be especially useful as we plan
to add a fourth format: changelog-v2.
Differential Revision: https://phab.mercurial-scm.org/D10622
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state