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

Since 0beee4c6 (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 7a26e653 (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: NJunio C Hamano <gitster@pobox.com>

builtin/apply.c

@@ -43,6 +43,7 @@ static int apply = 1;
 static int apply_in_reverse;
 static int apply_with_reject;
 static int apply_verbosely;
+static int allow_overlap;
 static int no_add;
 static const char *fake_ancestor;
 static int line_termination = '\n';
@@ -2430,9 +2431,9 @@ static void update_image(struct image *img,
 	memcpy(img->line + applied_pos,
 	       postimage->line,
 	       postimage->nr * sizeof(*img->line));
-	for (i = 0; i < postimage->nr; i++)
-		img->line[applied_pos + i].flag |= LINE_PATCHED;
-
+	if (!allow_overlap)
+		for (i = 0; i < postimage->nr; i++)
+			img->line[applied_pos + i].flag |= LINE_PATCHED;
 	img->nr = nr;
 }
 
@@ -3877,6 +3878,8 @@ int cmd_apply(int argc, const char **argv, const char *prefix_)
 			"don't expect at least one line of context"),
 		OPT_BOOLEAN(0, "reject", &apply_with_reject,
 			"leave the rejected hunks in corresponding *.rej files"),
+		OPT_BOOLEAN(0, "allow-overlap", &allow_overlap,
+			"allow overlapping hunks"),
 		OPT__VERBOSE(&apply_verbosely, "be verbose"),
 		OPT_BIT(0, "inaccurate-eof", &options,
 			"tolerate incorrectly detected missing new-line at the end of file",

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;
 }