pull: error on no merge candidates

pull: error on no merge candidates

Commit a8c9bef (pull: improve advice for unconfigured error case,
2009-10-05) fully established the current advices given by git-pull for
the different cases where git-fetch will not have anything marked for
merge:

1. We fetched from a specific remote, and a refspec was given, but it
   ended up not fetching anything. This is usually because the user
   provided a wildcard refspec which had no matches on the remote end.

2. We fetched from a non-default remote, but didn't specify a branch to
   merge. We can't use the configured one because it applies to the
   default remote, and thus the user must specify the branches to merge.

3. We fetched from the branch's or repo's default remote, but:

   a. We are not on a branch, so there will never be a configured branch
      to merge with.

   b. We are on a branch, but there is no configured branch to merge
      with.

4. We fetched from the branch's or repo's default remote, but the
   configured branch to merge didn't get fetched (either it doesn't
   exist, or wasn't part of the configured fetch refspec)

Re-implement the above behavior by implementing get_merge_heads() to
parse the heads in FETCH_HEAD for merging, and implementing
die_no_merge_candidates(), which will be called when FETCH_HEAD has no
heads for merging.

Helped-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>