"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>

git-add--interactive.perl

@@ -705,7 +705,7 @@ sub add_untracked_cmd {
 sub run_git_apply {
 	my $cmd = shift;
 	my $fh;
-	open $fh, '| git ' . $cmd . " --recount";
+	open $fh, '| git ' . $cmd . " --recount --allow-overlap";
 	print $fh @_;
 	return close $fh;
 }