commit.c: remove print_commit_list()
The helper function tries to offer a way to conveniently show the
last one differently from others, presumably to allow you to say
something like
A, B, and C.
while iterating over a list that has these three elements.
However, there is only one caller, and it passes the same format
string "%s\n" for both the last one and the other ones. Retire the
helper function and update the caller with a simplified version.
Signed-off-by: Junio C Hamano <gitster@pobox.com>