The mergetool--lib scriplet was tricky to use because it relied upon
the existance of several global shell variables. This removes more
global variables so that things are simpler for callers.
A side effect is that some variables are recomputed each time
run_merge_tool() is called, but the overhead for recomputing
them is justified by the simpler implementation.
Signed-off-by: David Aguilar <davvid@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>