fast-export: Make sure we show actual ref names instead of "(null)"

fast-export: Make sure we show actual ref names instead of "(null)"

The code expects a ref name to be provided in commit->util.  While there
was some code to set commit->util, it only worked in cases where there was
an unbroken chain of revisions from a ref to the relevant commit.  In
cases such as running
   git fast-export --parents master -- COPYING
commit->util would fail to be set.  The old method of setting commit->util
has been removed in favor of requesting show_source from the revision
traversal machinery (related to the "--source" option of "git log" family
of commands.)

However, this change does not fix cases like
   git fast export master~1
or
   git fast export :/arguments
since in such cases commit->util will be "master~1" or ":/arguments" while
we need the actual ref (e.g. "refs/heads/master")

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>