add -p: optimize line selection for short hunks

If there are fewer than ten changes in a hunk then make spaces
optional when selecting individual lines. This means that for short
hunks one can just type 1-357 to stage lines 1, 2, 3, 5 & 7.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>