merge: merge unborn index before setting ref

merge: merge unborn index before setting ref

When we merge into an unborn branch, there are basically two
steps:

  1. Write the sha1 of the new commit into the ref pointed
     to by HEAD.

  2. Update the index with the new content, and check it out
     to the working tree.

We currently do them in this order. However, (2) is the step
that is much more likely to fail, since it can be blocked by
things like untracked working tree files. When it does, the
merge fails and we are left with an empty index but an
updated HEAD.

This patch switches the order, so that a failure in updating
the index leaves us unchanged. Of course, a failure in
updating the ref now leaves us with an updated index and
mis-matched HEAD. That is arguably not much better, but it
is probably less likely to actually happen.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>