diff_unique_abbrev: rename to diff_aligned_abbrev

diff_unique_abbrev: rename to diff_aligned_abbrev

The word "align" describes how the function actually differs
from find_unique_abbrev, and will make it less confusing
when we add more diff-specific abbrevation functions that do
not do this alignment.

Since this is a globally available function, let's also move
its descriptive comment to the header file, where we
typically document function interfaces.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>