"add -p": work-around an old laziness that does not coalesce hunks

"add -p": work-around an old laziness that does not coalesce hunks

Since 0beee4c (git-add--interactive: remove hunk coalescing, 2008-07-02),
"git add--interactive" behaves lazily and passes overlapping hunks to the
underlying "git apply" without coalescing.  This was partially corrected
by 7a26e65 (its partial revert, 2009-05-16), but overlapping hunks are
still passed when the patch is edited.

Teach "git apply" the --allow-overlap option that disables a safety
feature that avoids misapplication of patches by not applying patches
to overlapping hunks, and pass this option form "add -p" codepath.

Do not even advertise the option, as this is merely a workaround, and the
correct fix is to make "add -p" correctly coalesce adjacent patch hunks.

Signed-off-by: Junio C Hamano <gitster@pobox.com>