commit.c: remove print_commit_list()

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>