diff: emit -L hunk headers via xdiff's formatter

The line-range filter builds its own "@@ -<old> +<new> @@" header for
each range hunk.  For a side with no lines (count 0, such as the old
side of a pure insertion), the begin should be the number of the line
before the change, per the convention git diff and xdl_emit_hunk_hdr()
follow.  The hand-rolled code's begin was one too high; in t4211 this
produced

	@@ -25,0 +18,9 @@

an old begin of 25 in a 24-line file, where git diff would give 24.

Stop hand-rolling the header.  flush_range_hunk() now formats it through
xdiff's own emitter: a new xdiff_emit_hunk_header() helper wraps
xdl_emit_hunk_hdr(), the function that produces every other diff's hunk
headers.  The count-0 begin is then correct by construction, and as a
side effect -L headers match git diff exactly, including its omission of
a count of 1 ("@@ -22 +22 @@" rather than "@@ -22,1 +22,1 @@").

xdiff's hunk callback already hands line_range_hunk_fn() a count-0 begin
decremented, so undo that when seeding the cursors and let the formatter
re-apply the convention once, at emit time.

The off-by-one predates this series, and the two regenerated fixtures
reach it from different origins: no-assertion-error has carried it since
its test was added in ab60c693a2 (line-log: fix assertion error,
2025-08-18), while vanishes-early acquired it when 86e986f166 (line-log:
route -L output through the standard diff pipeline) reshaped its tracked
line into a pure insertion.  vanishes-early also drops its count-1
counts.

Signed-off-by: Michael Montalbo <mmontalbo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -2636,14 +2636,9 @@ static void flush_range_hunk(struct line_range_filter *filter)
 		return;
 	}
 
-	strbuf_addf(&hdr, "@@ -%ld,%ld +%ld,%ld @@",
-		    filter->hunk.old_begin, old_count,
-		    filter->hunk.new_begin, new_count);
-	if (filter->funclen > 0) {
-		strbuf_addch(&hdr, ' ');
-		strbuf_add(&hdr, filter->func, filter->funclen);
-	}
-	strbuf_addch(&hdr, '\n');
+	xdiff_emit_hunk_header(&hdr, filter->hunk.old_begin, old_count,
+			       filter->hunk.new_begin, new_count,
+			       filter->func, filter->funclen);
 
 	filter->ret = filter->orig_line_fn(filter->orig_cb_data, hdr.buf, hdr.len);
 	strbuf_release(&hdr);
@@ -2668,19 +2663,21 @@ static void flush_range_hunk(struct line_range_filter *filter)
 }
 
 static void line_range_hunk_fn(void *data,
-			       long old_begin, long old_nr UNUSED,
-			       long new_begin, long new_nr UNUSED,
+			       long old_begin, long old_nr,
+			       long new_begin, long new_nr,
 			       const char *func, long funclen)
 {
 	struct line_range_filter *filter = data;
 
 	/*
-	 * When count > 0, begin is 1-based.  When count == 0, begin is
-	 * adjusted down by 1 by xdl_emit_hunk_hdr(), but no lines of
-	 * that type will arrive, so the value is unused.
+	 * Seed the per-image line cursors from the hunk header's begins.  For
+	 * a side with no lines (count 0), xdiff's callback has already moved
+	 * its begin to the line before the change, so add one back to recover
+	 * the true 1-based start.  xdiff_emit_hunk_header() reapplies that -1
+	 * when the clipped hunk is emitted.
 	 */
-	filter->lno_in_postimage = new_begin;
-	filter->lno_in_preimage = old_begin;
+	filter->lno_in_postimage = new_nr ? new_begin : new_begin + 1;
+	filter->lno_in_preimage = old_nr ? old_begin : old_begin + 1;
 
 	if (funclen > 0) {
 		if (funclen > (long)sizeof(filter->func))

t/t4211/sha1/expect.no-assertion-error

@@ -8,7 +8,7 @@ diff --git a/b.c b/b.c
 index bf79c2f..27c829c 100644
 --- a/b.c
 +++ b/b.c
-@@ -25,0 +18,9 @@
+@@ -24,0 +18,9 @@
 +long f(long x)
 +{
 +	int s = 0;

t/t4211/sha1/expect.vanishes-early

@@ -8,7 +8,7 @@ diff --git a/a.c b/a.c
 index 0b9cae5..5de3ea4 100644
 --- a/a.c
 +++ b/a.c
-@@ -23,0 +24,1 @@ int main ()
+@@ -22,0 +24 @@ int main ()
 +/* incomplete lines are bad! */
 
 commit 100b61a6f2f720f812620a9d10afb3a960ccb73c
@@ -21,7 +21,7 @@ diff --git a/a.c b/a.c
 index 5e709a1..0b9cae5 100644
 --- a/a.c
 +++ b/a.c
-@@ -22,1 +22,1 @@ int main ()
+@@ -22 +22 @@ int main ()
 -}
 +}
 \ No newline at end of file
@@ -37,5 +37,5 @@ new file mode 100644
 index 0000000..444e415
 --- /dev/null
 +++ b/a.c
-@@ -0,0 +20,1 @@
+@@ -0,0 +20 @@
 +}

t/t4211/sha256/expect.no-assertion-error

@@ -8,7 +8,7 @@ diff --git a/b.c b/b.c
 index 69cb69c..a0d566e 100644
 --- a/b.c
 +++ b/b.c
-@@ -25,0 +18,9 @@
+@@ -24,0 +18,9 @@
 +long f(long x)
 +{
 +	int s = 0;

t/t4211/sha256/expect.vanishes-early

@@ -8,7 +8,7 @@ diff --git a/a.c b/a.c
 index e4fa1d8..62c1fc2 100644
 --- a/a.c
 +++ b/a.c
-@@ -23,0 +24,1 @@ int main ()
+@@ -22,0 +24 @@ int main ()
 +/* incomplete lines are bad! */
 
 commit 29f32ac3141c48b22803e5c4127b719917b67d0f8ca8c5248bebfa2a19f7da10
@@ -21,7 +21,7 @@ diff --git a/a.c b/a.c
 index d325124..e4fa1d8 100644
 --- a/a.c
 +++ b/a.c
-@@ -22,1 +22,1 @@ int main ()
+@@ -22 +22 @@ int main ()
 -}
 +}
 \ No newline at end of file
@@ -37,5 +37,5 @@ new file mode 100644
 index 0000000..9f550c3
 --- /dev/null
 +++ b/a.c
-@@ -0,0 +20,1 @@
+@@ -0,0 +20 @@
 +}

xdiff-interface.c

@@ -91,6 +91,25 @@ static int xdiff_outf(void *priv_, mmbuffer_t *mb, int nbuf)
 	return 0;
 }
 
+static int strbuf_out_line(void *priv, mmbuffer_t *mb, int nbuf)
+{
+	struct strbuf *out = priv;
+	int i;
+	for (i = 0; i < nbuf; i++)
+		strbuf_add(out, mb[i].ptr, mb[i].size);
+	return 0;
+}
+
+void xdiff_emit_hunk_header(struct strbuf *out,
+			    long old_begin, long old_count,
+			    long new_begin, long new_count,
+			    const char *func, long funclen)
+{
+	xdemitcb_t ecb = { .priv = out, .out_line = strbuf_out_line };
+	xdl_emit_hunk_hdr(old_begin, old_count, new_begin, new_count,
+			  func, funclen, &ecb);
+}
+
 /*
  * Trim down common substring at the end of the buffers,
  * but end on a complete line.

xdiff-interface.h

@@ -76,4 +76,19 @@ int xdiff_compare_lines(const char *l1, long s1,
  */
 unsigned long xdiff_hash_string(const char *s, size_t len, long flags);
 
+struct strbuf;
+
+/*
+ * Append a unified-diff hunk header to `out`, e.g.
+ * "@@ -<old> +<new> @@ func\n".  The header comes from wrapping xdiff's
+ * own hunk-header emitter, so it matches what a normal diff would
+ * produce for these begins and counts.  For a side with no lines
+ * (count 0) the begin is the line before the change, and a count of 1
+ * is omitted.
+ */
+void xdiff_emit_hunk_header(struct strbuf *out,
+			    long old_begin, long old_count,
+			    long new_begin, long new_count,
+			    const char *func, long funclen);
+
 #endif