sha1_name: tweak @{-N} lookup

sha1_name: tweak @{-N} lookup

Have the lookup only look at "interesting" checkouts, meaning those
that tell you "Already on ..." don't count even though they also cause
a reflog entry.

Let interpret_nth_last_branch() return the number of characters
parsed, so that git-checkout can verify that the branch spec was
@{-N}, not @{-1}^2 or something like that.  (The latter will be added
later.)

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