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

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

Even though git makes sure that it uses enough hexdigits to show an
abbreviated object name unambiguously, as more objects are added to the
repository over time, a short name that used to be unique will stop being
unique.  Git uses this many extra hexdigits that are more than necessary
to make the object name currently unique, in the hope that its output will
stay unique a bit longer.

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