Documentation: extract script to generate a list of mergetools

Documentation: extract script to generate a list of mergetools

We include the list of available mergetools into our manpages. Extract
the script that performs this logic such that we can reuse it in other
build systems.

While at it, refactor the Makefile targets such that we don't create
"mergetools-list.made" anymore. It shouldn't be necessary, as we can
instead have other targets depend on "mergetools-{diff,merge}.txt"
directly.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>