mergetools/diffmerge: support DiffMerge as a git mergetool

DiffMerge is a non-free (but gratis) tool that supports OS X, Windows and Linux.

    See http://www.sourcegear.com/diffmerge/



DiffMerge includes a script `/usr/bin/diffmerge` that can be used to launch the
graphical compare tool.

This change adds mergetool support for DiffMerge and adds 'diffmerge' as an
option to the mergetool help.

Signed-off-by: Stefan Saasen <ssaasen@atlassian.com>
Acked-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>