pull: check if in unresolved merge state

pull: check if in unresolved merge state

Since d38a30d (Be more user-friendly when refusing to do something
because of conflict., 2010-01-12), git-pull will error out with
user-friendly advices if the user is in the middle of a merge or has
unmerged files.

Re-implement this behavior. While the "has unmerged files" case can be
handled by die_resolve_conflict(), we introduce a new function
die_conclude_merge() for printing a different error message for when
there are no unmerged files but the merge has not been finished.

Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>