emitrevision: simplify the fallback to computed delta
Not using the stored delta, or having a full snapshot on disk behave the same
ways, so lets use the same code path for that, this is simpler, and it update
will be simpler.
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init repo
$ cd repo
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state