[PATCH] git-add -p: be able to undo a given hunk

@@ -254,6 +254,7 @@ patch::
 
        y - stage this hunk
        n - do not stage this hunk
+       u - do not stage this hunk and revert it
        q - quit, do not stage this hunk nor any of the remaining ones
        a - stage this and all the remaining hunks in the file
        d - do not stage this hunk nor any of the remaining hunks in the file

@@ -693,6 +693,7 @@ sub split_hunk {
 			ADDDEL => 0,
 			POSTCTX => 0,
 			USE => undef,
+			UNDO => undef,
 		};
 
 		while (++$i < @$text) {

@@ -835,12 +836,13 @@ sub merge_hunk {
 }
 
 sub coalesce_overlapping_hunks {
+	my $field = shift;
 	my (@in) = @_;
 	my @out = ();
 
 	my ($last_o_ctx, $last_was_dirty);
 
-	for (grep { $_->{USE} } @in) {
+	for (grep { $_->{$field} } @in) {
 		my $text = $_->{TEXT};
 		my ($o_ofs) = parse_hunk_header($text->[0]);
 		if (defined $last_o_ctx &&

@@ -991,6 +993,7 @@ sub help_patch_cmd {
 	print colored $help_color, <<\EOF ;
 y - stage this hunk
 n - do not stage this hunk
+u - do not stage this hunk and revert it
 q - quit, do not stage this hunk nor any of the remaining ones
 a - stage this and all the remaining hunks in the file
 d - do not stage this hunk nor any of the remaining hunks in the file

@@ -1140,7 +1143,7 @@ sub patch_update_file {
 		}
 		print colored $prompt_color, 'Stage ',
 		  ($hunk[$ix]{TYPE} eq 'mode' ? 'mode change' : 'this hunk'),
-		  " [y,n,q,a,d,/$other,?]? ";
+		  " [y,n,u,q,a,d,/$other,?]? ";
 		my $line = prompt_single_character;
 		if ($line) {
 			if ($line =~ /^y/i) {

@@ -1149,6 +1152,10 @@ sub patch_update_file {
 			elsif ($line =~ /^n/i) {
 				$hunk[$ix]{USE} = 0;
 			}
+			elsif ($line =~ /^u/) {
+				$hunk[$ix]{USE} = 0;
+				$hunk[$ix]{UNDO} = 1;
+			}
 			elsif ($line =~ /^a/i) {
 				while ($ix < $num) {
 					if (!defined $hunk[$ix]{USE}) {

@@ -1301,14 +1308,14 @@ sub patch_update_file {
 		}
 	}
 
-	@hunk = coalesce_overlapping_hunks(@hunk);
-
 	my $n_lofs = 0;
 	my @result = ();
-	for (@hunk) {
-		if ($_->{USE}) {
-			push @result, @{$_->{TEXT}};
-		}
+	my @undo = ();
+	for (coalesce_overlapping_hunks("USE", @hunk)) {
+		push @result, @{$_->{TEXT}};
+	}
+	for (coalesce_overlapping_hunks("UNDO", @hunk)) {
+		push @undo, @{$_->{TEXT}};
 	}
 
 	if (@result) {

@@ -1326,6 +1333,21 @@ sub patch_update_file {
 		refresh();
 	}
 
+	if (@undo) {
+		my $fh;
+
+		open $fh, '| git apply -R';
+		for (@{$head->{TEXT}}, @undo) {
+			print $fh $_;
+		}
+		if (!close $fh) {
+			for (@{$head->{TEXT}}, @undo) {
+				print STDERR $_;
+			}
+		}
+		refresh();
+	}
+
 	print "\n";
 	return $quit;
 }