mergetool--lib: create gui_mode function

mergetool--lib: create gui_mode function

Before, in `get_configured_merge_tool`, we would test the value of the
first argument directly, which corresponded to whether we were using
guitool. However, since `$GIT_MERGETOOL_GUI` is available as an
environment variable, create the `gui_mode` function which increases the
clarify of functions which use it.

While we're at it, add a space before `()` in function definitions to
fix the style.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>