Test for recent rev-parse $abbrev_sha1 regression

Test for recent rev-parse $abbrev_sha1 regression

My recent patch "Lazily open pack index files on demand" caused a
regression in the case of parsing abbreviated SHA-1 object names.
Git was unable to translate the abbreviated name into the full name
if the object was packed, as the pack .idx files were not opened
before being accessed.

This is a simple test to repack a repository then test for an
abbreviated SHA-1 within the packfile.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>