Add test cases for git-am

Add test cases for git-am

Add t/t4151-am.sh that does basic testing of git-am functionality,
including:
 * am applies patch correctly
 * am changes committer and keeps author
 * am --signoff adds Signed-off-by: line
 * am stays in branch
 * am --signoff does not add Signed-off-by: line if already there
 * am without --keep removes Re: and [PATCH] stuff
 * am --keep really keeps the subject
 * am -3 falls back to 3-way merge
 * am pauses on conflict
 * am --skip works
 * am --resolved works
 * am takes patches from a Pine mailbox
 * am fails on mail without patch
 * am fails on empty patch

Signed-off-by: Stephan Beyer <s-beyer@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>