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
#require docutils gettext
$ $TESTDIR/check-gendoc sv
checking for parse errors