log: do not print ellipses with --abbrev-commit

log: do not print ellipses with --abbrev-commit

'git log --abbrev-commit' added an ellipsis to all commit names that
were abbreviated.  This was particularly annoying if you wanted to
cut&paste the sha1 from the terminal, since selecting by word would
pick up '...'  too.

So use find_unique_abbrev() instead of diff_unique_abbrev() in all
log-related commit sha1 printing routines, and also change the
formatting of the 'Merge: parent1 parent2' line output via
pretty_print_commit().

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>