add -p: fix counting empty context lines in edited patches

recount_edited_hunk() introduced in commit 2b8ea7f3c7 ("add -p:
calculate offset delta for edited patches", 2018-03-05) required all
context lines to start with a space, empty lines are not counted. This
was intended to avoid any recounting problems if the user had
introduced empty lines at the end when editing the patch. However this
introduced a regression into 'git add -p' as it seems it is common for
editors to strip the trailing whitespace from empty context lines when
patches are edited thereby introducing empty lines that should be
counted. 'git apply' knows how to deal with such empty lines and POSIX
states that whether or not there is an space on an empty context line
is implementation defined [1].

Fix the regression by counting lines that consist solely of a newline
as well as lines starting with a space as context lines and add a test
to prevent future regressions.

[1] http://pubs.opengroup.org/onlinepubs/9699919799/utilities/diff.html

Reported-by: Mahmoud Al-Qudsi <mqudsi@neosmart.net>
Reported-by: Oliver Joseph Ash <oliverjash@gmail.com>
Reported-by: Jeff Felchner <jfelchner1@gmail.com>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-add--interactive.perl

@@ -1047,7 +1047,7 @@ sub recount_edited_hunk {
 			$o_cnt++;
 		} elsif ($mode eq '+') {
 			$n_cnt++;
-		} elsif ($mode eq ' ') {
+		} elsif ($mode eq ' ' or $mode eq "\n") {
 			$o_cnt++;
 			$n_cnt++;
 		}

t/t3701-add-interactive.sh

@@ -175,6 +175,49 @@ test_expect_success 'real edit works' '
 	diff_cmp expected output
 '
 
+test_expect_success 'setup file' '
+	test_write_lines a "" b "" c >file &&
+	git add file &&
+	test_write_lines a "" d "" c >file
+'
+
+test_expect_success 'setup patch' '
+	SP=" " &&
+	NULL="" &&
+	cat >patch <<-EOF
+	@@ -1,4 +1,4 @@
+	 a
+	$NULL
+	-b
+	+f
+	$SP
+	c
+	EOF
+'
+
+test_expect_success 'setup expected' '
+	cat >expected <<-EOF
+	diff --git a/file b/file
+	index b5dd6c9..f910ae9 100644
+	--- a/file
+	+++ b/file
+	@@ -1,5 +1,5 @@
+	 a
+	$SP
+	-f
+	+d
+	$SP
+	 c
+	EOF
+'
+
+test_expect_success 'edit can strip spaces from empty context lines' '
+	test_write_lines e n q | git add -p 2>error &&
+	test_must_be_empty error &&
+	git diff >output &&
+	diff_cmp expected output
+'
+
 test_expect_success 'skip files similarly as commit -a' '
 	git reset &&
 	echo file >.gitignore &&