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.