mergetool--lib: Refactor tools into separate files

Individual merge tools are now defined in a mergetools/$tool
file which is sourced at runtime.

The individual files are installed into $(git --exec-path)/mergetools/.
New tools can be added by creating a new file instead of editing the
git-mergetool--lib.sh scriptlet.

http://thread.gmane.org/gmane.comp.version-control.git/134906/focus=135006



Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>