test: refactor to use "get_abbrev_oid" to get abbrev oid

test: refactor to use "get_abbrev_oid" to get abbrev oid

Add new function "get_abbrev_oid" to get abbrev object ID.  This
function has a default value which helps to prepare a nonempty replace
pattern for sed command.  An empty replace pattern may cause sed fail
to allocate memory.

Refactor function "make_user_friendly_and_stable_output" to use
"get_abbrev_oid" to get abbrev object ID.

Signed-off-by: Jiang Xin <zhiyou.jx@alibaba-inc.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>