Revert "core.abbrevguard: Ensure short object names stay unique a bit longer"

This reverts commit 72a5b561fc1c4286bc7c5b0693afc076af261e1f, as adding
fixed number of hexdigits more than necessary to make one object name
locally unique does not help in futureproofing the uniqueness of names
we generate today.

Signed-off-by: Junio C Hamano <gitster@pobox.com>