remove blank filename in error message

remove blank filename in error message

When write_loose_object() finds that it is unable to
create a temporary file, it complains, for instance:

    unable to create temporary sha1 filename : Too many open files

That extra space was supposed to be the name of the file,
and will be an empty string if the git_mkstemps_mode() fails.

The name of the temporary file is unimportant; delete it.

Signed-off-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>