git-merge-resolve.sh

Let the merge strategies handle the base less case if they are able to
do it. It also fixes git-resolve.sh to die if no common ancestors
exists, instead of doing the wrong thing. Furthermore, it contains a
small independent fix for git-merge.sh and a fix for a base less code
path in gitMergeCommon.py.

With this it's possible to use
    git merge -s recursive 'merge message' A B
to do a base less merge of A and B.

[jc: Thanks Fredrik for fixing the brown-paper-bag in git-merge.
 I fixed a small typo in git-merge-resolve fix; 'test' equality
 check is spelled with single equal sign -- C-style double equal
 sign is bashism.]

Signed-off-by: Fredrik Kuivinen <freku045@student.liu.se>
Signed-off-by: Junio C Hamano <junkio@cox.net>