Revert "Allow branch.*.merge to talk about remote tracking branches."

Revert "Allow branch.*.merge to talk about remote tracking branches."

This reverts commit 80c797764a6b6a373f0f1f47d7f56b0d950418a9.

Back when I committed this, it seemed to be a good idea.  People
who always use remote tracking branches can optionally use the
local name they happen to use to specify what to merge, which meant
that I did not have to teach them why we use the name at the remote
side every time they are confused.

But allowing it seems to break other people's scripts.  The real
solution is not to allow more ways to express the same thing, but
to educate people to use the right syntax.

Signed-off-by: Junio C Hamano <junkio@cox.net>