merge: respect ui.relative-paths
We print file paths in a lot of places in this code and I've probably
missed a few places. We can fix them as we discover them (I'm also
happy to fix anything reviewers notice, of course).
Differential Revision: https://phab.mercurial-scm.org/D5747
$ 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