Teach fsck and prune that tmp_obj_ file names may not be 14 bytes long

Teach fsck and prune that tmp_obj_ file names may not be 14 bytes long

As Shawn pointed out, not all temporary file creation routines can
ensure that the generated temporary file is of a certain length.
e.g. Java's createTempFile(prefix, suffix). So just depend on the
prefix 'tmp_obj_' for detection.

Update prune, and fix the "fix" introduced by a08c53a1 :)

Signed-off-by: Brandon "appendixless" Casey <casey@nrlssc.navy.mil>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>