[PATCH 2/3] diff-parseopt: restore -U (no argument) behavior

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

Before d473e2e0e8 (diff.c: convert -U|--unified, 2019-01-27), -U and
--unified are implemented with a custom parser opt_arg() in diff.c. I
didn't check this code carefully and not realize that it's the
equivalent of PARSE_OPT_NONEG | PARSE_OPT_OPTARG.

In other words, if -U is specified without any argument, the option
should be accepted, and the default value should be used. Without
PARSE_OPT_OPTARG, parse_options() will reject this case and cause a
regression.

Reported-by: Bryan Turner <bturner@atlassian.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 diff.c                                    | 10 ++++---
 t/t4013-diff-various.sh                   |  2 ++
 t/t4013/diff.diff_-U1_initial..side (new) | 29 ++++++++++++++++++++
 t/t4013/diff.diff_-U2_initial..side (new) | 31 ++++++++++++++++++++++
 t/t4013/diff.diff_-U_initial..side (new)  | 32 +++++++++++++++++++++++
 5 files changed, 100 insertions(+), 4 deletions(-)

diff --git a/diff.c b/diff.c
index 4d3cf83a27..80ddc11671 100644
--- a/diff.c
+++ b/diff.c
@@ -5211,9 +5211,11 @@ static int diff_opt_unified(const struct option *opt,
 
 	BUG_ON_OPT_NEG(unset);
 
-	options->context = strtol(arg, &s, 10);
-	if (*s)
-		return error(_("%s expects a numerical value"), "--unified");
+	if (arg) {
+		options->context = strtol(arg, &s, 10);
+		if (*s)
+			return error(_("%s expects a numerical value"), "--unified");
+	}
 	enable_patch_output(&options->output_format);
 
 	return 0;
@@ -5272,7 +5274,7 @@ static void prep_parse_options(struct diff_options *options)
 			  DIFF_FORMAT_PATCH, DIFF_FORMAT_NO_OUTPUT),
 		OPT_CALLBACK_F('U', "unified", options, N_("<n>"),
 			       N_("generate diffs with <n> lines context"),
-			       PARSE_OPT_NONEG, diff_opt_unified),
+			       PARSE_OPT_NONEG | PARSE_OPT_OPTARG, diff_opt_unified),
 		OPT_BOOL('W', "function-context", &options->flags.funccontext,
 			 N_("generate diffs with <n> lines context")),
 		OPT_BIT_F(0, "raw", &options->output_format,
diff --git a/t/t4013-diff-various.sh b/t/t4013-diff-various.sh
index 9f8f0e84ad..a9054d2db1 100755
--- a/t/t4013-diff-various.sh
+++ b/t/t4013-diff-various.sh
@@ -338,6 +338,8 @@ format-patch --inline --stdout initial..master^^
 format-patch --stdout --cover-letter -n initial..master^
 
 diff --abbrev initial..side
+diff -U initial..side
+diff -U1 initial..side
 diff -r initial..side
 diff --stat initial..side
 diff -r --stat initial..side
diff --git a/t/t4013/diff.diff_-U1_initial..side b/t/t4013/diff.diff_-U1_initial..side
new file mode 100644
index 0000000000..b69f8f048a
--- /dev/null
+++ b/t/t4013/diff.diff_-U1_initial..side
@@ -0,0 +1,29 @@
+$ git diff -U1 initial..side
+diff --git a/dir/sub b/dir/sub
+index 35d242b..7289e35 100644
+--- a/dir/sub
++++ b/dir/sub
+@@ -2 +2,3 @@ A
+ B
++1
++2
+diff --git a/file0 b/file0
+index 01e79c3..f4615da 100644
+--- a/file0
++++ b/file0
+@@ -3 +3,4 @@
+ 3
++A
++B
++C
+diff --git a/file3 b/file3
+new file mode 100644
+index 0000000..7289e35
+--- /dev/null
++++ b/file3
+@@ -0,0 +1,4 @@
++A
++B
++1
++2
+$
diff --git a/t/t4013/diff.diff_-U2_initial..side b/t/t4013/diff.diff_-U2_initial..side
new file mode 100644
index 0000000000..8ffe04f203
--- /dev/null
+++ b/t/t4013/diff.diff_-U2_initial..side
@@ -0,0 +1,31 @@
+$ git diff -U2 initial..side
+diff --git a/dir/sub b/dir/sub
+index 35d242b..7289e35 100644
+--- a/dir/sub
++++ b/dir/sub
+@@ -1,2 +1,4 @@
+ A
+ B
++1
++2
+diff --git a/file0 b/file0
+index 01e79c3..f4615da 100644
+--- a/file0
++++ b/file0
+@@ -2,2 +2,5 @@
+ 2
+ 3
++A
++B
++C
+diff --git a/file3 b/file3
+new file mode 100644
+index 0000000..7289e35
+--- /dev/null
++++ b/file3
+@@ -0,0 +1,4 @@
++A
++B
++1
++2
+$
diff --git a/t/t4013/diff.diff_-U_initial..side b/t/t4013/diff.diff_-U_initial..side
new file mode 100644
index 0000000000..c66c0dd5c6
--- /dev/null
+++ b/t/t4013/diff.diff_-U_initial..side
@@ -0,0 +1,32 @@
+$ git diff -U initial..side
+diff --git a/dir/sub b/dir/sub
+index 35d242b..7289e35 100644
+--- a/dir/sub
++++ b/dir/sub
+@@ -1,2 +1,4 @@
+ A
+ B
++1
++2
+diff --git a/file0 b/file0
+index 01e79c3..f4615da 100644
+--- a/file0
++++ b/file0
+@@ -1,3 +1,6 @@
+ 1
+ 2
+ 3
++A
++B
++C
+diff --git a/file3 b/file3
+new file mode 100644
+index 0000000..7289e35
+--- /dev/null
++++ b/file3
+@@ -0,0 +1,4 @@
++A
++B
++1
++2
+$
-- 
2.22.0.rc0.322.g2b0371e29a