meson: fix generation of merge tools

meson: fix generation of merge tools

Our buildsystems generate a list of diff and merge tools that ultimately
end up in our documentation. And while Meson does wire up the logic, it
tries to use the TOOL_MODE environment variable to set up the mode. This
is wrong though: the mode is set via an argument that we have fixed to
'diff' mode by accident.

Fix this such that merge tools are properly generated.

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