commit-graph.c: don't use discarded graph_name in error

commit-graph.c: don't use discarded graph_name in error

When writing a commit-graph layer, we do so in a temporary file which is
renamed into place. If we fail to create a temporary file, for e.g.,
because we have too many open files, then 'git_mkstemp_mode' sets the
pattern to the empty string, in which case we get an error something
along the lines of:

  error: unable to create ''

It's not useful to show the pattern here at all, since we (1) know the
pattern is well-formed, and (2) would have already shown the dirname
when trying to create the leading directories. So, replace this error
with something friendlier.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>