write first for-merge ref to FETCH_HEAD first

write first for-merge ref to FETCH_HEAD first

The FETCH_HEAD refname is supposed to refer to the ref that was fetched
and should be merged. However all fetched refs are written to
.git/FETCH_HEAD in an arbitrary order, and resolve_ref_unsafe simply
takes the first ref as the FETCH_HEAD, which is often the wrong one,
when other branches were also fetched.

The solution is to write the for-merge ref(s) to FETCH_HEAD first.
Then, unless --append is used, the FETCH_HEAD refname behaves as intended.
If the user uses --append, they presumably are doing so in order to
preserve the old FETCH_HEAD.

While we are at it, update an old example in the read-tree documentation
that implied that each entry in FETCH_HEAD only has the object name, which
is not true for quite a while.

[jc: adjusted tests]

Signed-off-by: Joey Hess <joey@kitenet.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>