[PATCH v2 3/4] add -p: handle `diff-so-fancy`'s hunk headers better

["Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

The `diff-so-fancy` diff colorizer produces hunk headers that look
nothing like the built-in `add -p` expects: there is no `@@ ... @@` line
range, and therefore the parser cannot determine where any extra
information starts, such as the function name that is often added to
those hunk header lines.

However, we can do better than simply swallowing the unparseable hunk
header. In the `diff-so-fancy` case, it shows something like `@ file:1
@`. Let's just show the complete hunk header because it probably offers
useful information.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 add-patch.c                | 8 +++++++-
 t/t3701-add-interactive.sh | 3 ++-
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/add-patch.c b/add-patch.c
index f2fffe1af02..1f3f3611ee9 100644
--- a/add-patch.c
+++ b/add-patch.c
@@ -8,6 +8,7 @@
 #include "diff.h"
 #include "compat/terminal.h"
 #include "prompt.h"
+#include "utf8.h"
 
 enum prompt_mode_type {
 	PROMPT_MODE_CHANGE = 0, PROMPT_DELETION, PROMPT_ADDITION, PROMPT_HUNK,
@@ -363,7 +364,7 @@ static int parse_hunk_header(struct add_p_state *s, struct hunk *hunk)
 		header->colored_extra_start = p + 3 - s->colored.buf;
 	else
 		/* could not parse colored hunk header, showing nothing */
-		header->colored_extra_start = hunk->colored_start;
+		header->colored_extra_start = line - s->colored.buf;
 	header->colored_extra_end = hunk->colored_start;
 
 	return 0;
@@ -649,6 +650,7 @@ static void render_hunk(struct add_p_state *s, struct hunk *hunk,
 		size_t len;
 		unsigned long old_offset = header->old_offset;
 		unsigned long new_offset = header->new_offset;
+		int needs_extra_space = 0;
 
 		if (!colored) {
 			p = s->plain.buf + header->extra_start;
@@ -658,6 +660,8 @@ static void render_hunk(struct add_p_state *s, struct hunk *hunk,
 			p = s->colored.buf + header->colored_extra_start;
 			len = header->colored_extra_end
 				- header->colored_extra_start;
+			if (utf8_strnwidth(p, len, 1 /* skip ANSI */) > 0)
+				needs_extra_space = 1;
 		}
 
 		if (s->mode->is_reverse)
@@ -673,6 +677,8 @@ static void render_hunk(struct add_p_state *s, struct hunk *hunk,
 			strbuf_addf(out, ",%lu", header->new_count);
 		strbuf_addstr(out, " @@");
 
+		if (needs_extra_space)
+			strbuf_addch(out, ' ');
 		if (len)
 			strbuf_add(out, p, len);
 		else if (colored)
diff --git a/t/t3701-add-interactive.sh b/t/t3701-add-interactive.sh
index 7e3c1de71f5..9deb7a87f1e 100755
--- a/t/t3701-add-interactive.sh
+++ b/t/t3701-add-interactive.sh
@@ -772,7 +772,8 @@ test_expect_success 'gracefully fail to parse colored hunk header' '
 	echo content >test &&
 	test_config interactive.diffFilter "sed s/@@/XX/g" &&
 	printf y >y &&
-	force_color git add -p <y
+	force_color git add -p >output 2>&1 <y &&
+	grep XX output
 '
 
 test_expect_success 'diff.algorithm is passed to `git diff-files`' '
-- 
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