use strbuf_add_unique_abbrev() for adding short hashes, part 3

use strbuf_add_unique_abbrev() for adding short hashes, part 3

Call strbuf_add_unique_abbrev() to add abbreviated hashes to strbufs
instead of taking detours through find_unique_abbrev() and its static
buffer.  This is shorter in most cases and a bit more efficient.

The changes here are not easily handled by a semantic patch because
they involve removing temporary variables and deconstructing format
strings for strbuf_addf().

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>