extdiff: prepare sections of dodiff() for conditionalizing
This is purely indenting under an unconditional branch, so that the actual
changes in the next patch are clear. Feel free to fold them if desired.
$ 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