built-in add -p: coalesce hunks after splitting them

built-in add -p: coalesce hunks after splitting them

This is considered "the right thing to do", according to 933e44d3a0
("add -p": work-around an old laziness that does not coalesce hunks,
2011-04-06).

Note: we cannot simply modify the hunks while merging them; Once we
implement hunk editing, we will call `reassemble_patch()` whenever a
hunk is edited, therefore we must not modify the hunks (because the user
might e.g. hit `K` and change their mind whether to stage the previous
hunk).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>