getbundle: send highest changegroup format supported by both side
When using bundle2, we find the common subset of supported changegroup-packers
and we pick the max of them. This allow to use generaldelta aware changegroup
through bundle2.
$ 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