mergetools: add --nofork option to gvimdiff.diffargs for extdiff Without --nofork, temporary files are removed immediately before gvimdiff starts. "-d -g -O" are put just for consistency with gvimdiff.args.

contrib/vagrant: use Vagrant for running tests on virtual machine $ cd contrib/vagrant $ vagrant up $ vagrant ssh -c ./run-tests.sh Repository is shared at /hgshared in guest machine.