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