mergetool: run prompt only if guessed tool

It's annoying to see the prompt:

Hit return to start merge resolution tool (foo):

Every time the user does 'git mergetool' even if the user already
configured 'foo' as the wanted tool.

Display this prompt only when the user hasn't explicitly configured a
tool.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Acked-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>