Clean up find_unique_abbrev() callers

Clean up find_unique_abbrev() callers

Now find_unique_abbrev() never returns NULL, there is no need for callers
to prepare for seeing NULL and fall back to giving the full 40-hexdigits.

While we are at it, drop "..." in the "git reset" output that reports the
location of the new HEAD, between the abbreviated commit object name and
the one line commit summary.  Because we are always showing the HEAD
(which cannot be missing!), we never had a case where we show the full 40
hexdigits that is not followed by three dots, and these three dots were
stealing 3 columns from the precious horizontal screen real estate out of
80 that can better be used for the one line commit summary.

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