git-fetch: do not fail when remote branch disappears

git-fetch: do not fail when remote branch disappears

When the branch named with branch.$name.merge is not covered by
the fetch configuration for the remote repository named with
branch.$name.remote, we automatically add that branch to the set
of branches to be fetched.  However, if the remote repository
does not have that branch (e.g. it used to exist, but got
removed), this is not a reason to fail the git-fetch itself.

The situation however will be noticed if git-fetch was called by
git-pull, as the resulting FETCH_HEAD would not have any entry
that is marked for merging.

Acked-By: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>