Fix output headers in combinediff

Makefile.am

@@ -113,6 +113,7 @@ TESTS = tests/newline1/run-test \
 	tests/combine1/run-test \
 	tests/combine2/run-test \
 	tests/combine3/run-test \
+	tests/combine4/run-test \
 	tests/gendiff1/run-test \
 	tests/gendiff2/run-test \
 	tests/comma/run-test \

src/interdiff.c

@@ -1055,16 +1055,31 @@ output_delta (FILE *p1, FILE *p2, FILE *out)
 	tmpp1fd = xmkstemp (tmpp1);
 	tmpp2fd = xmkstemp (tmpp2);
 
-	do {
-		if (oldname) {
-			free (oldname);
-			oldname = NULL;
-		}
-		if (getline (&oldname, &namelen, p1) < 0)
-			error (EXIT_FAILURE, errno, "Bad patch #1");
+	if (mode == mode_combine) {
+		/* For combinediff, we want the --- line from patch1 (original file) */
+		do {
+			if (oldname) {
+				free (oldname);
+				oldname = NULL;
+			}
+			if (getline (&oldname, &namelen, p1) < 0)
+				error (EXIT_FAILURE, errno, "Bad patch #1");
 
-	} while (strncmp (oldname, "+++ ", 4));
-	oldname[strlen (oldname) - 1] = '\0';
+		} while (strncmp (oldname, "--- ", 4));
+		oldname[strlen (oldname) - 1] = '\0';
+	} else {
+		/* For interdiff, use +++ line from patch1 */
+		do {
+			if (oldname) {
+				free (oldname);
+				oldname = NULL;
+			}
+			if (getline (&oldname, &namelen, p1) < 0)
+				error (EXIT_FAILURE, errno, "Bad patch #1");
+
+		} while (strncmp (oldname, "+++ ", 4));
+		oldname[strlen (oldname) - 1] = '\0';
+	}
 
 	do {
 		if (newname) {

tests/combine4/run-test

@@ -0,0 +1,46 @@
+#!/bin/bash
+
+# This is a combinediff(1) testcase.
+# Bug report: combinediff produces incorrect file path prefixes
+# Expected: diff -u a/file.c a/file.c
+# Actual:   diff -u b/file.c b/file.c
+#
+# This test expects the correct behavior and will fail until the bug is fixed.
+# It's added to XFAIL_TESTS to mark it as an expected failure.
+
+. ${top_srcdir-.}/tests/common.sh
+
+# Create the patch files from the bug report
+cat << EOF > first.patch
+--- a/file.c
++++ b/file.c
+@@ -0,0 +1,3 @@
++a
++b
++c
+EOF
+
+cat << EOF > second.patch
+--- a/file.c
++++ b/file.c
+@@ -1,3 +1,2 @@
+ a
+ b
+-c
+EOF
+
+# Run combinediff with -p1 as mentioned in the bug report
+${COMBINEDIFF} -p1 first.patch second.patch 2>errors >combined.patch || exit 1
+[ -s errors ] && exit 1
+
+# Check that the combined patch exists and is not empty
+[ -s combined.patch ] || exit 1
+
+# Check that the combined patch has the correct header format
+head -3 combined.patch > header
+cat << EOF > expected
+diff -u a/file.c b/file.c
+--- a/file.c
++++ b/file.c
+EOF
+cmp header expected || exit 1

tests/trimlast2/run-test

@@ -35,8 +35,8 @@ ${COMBINEDIFF} patch1 patch2 > patch12 2>errors || exit 1
 [ -s errors ] && exit 1
 
 cat << EOF | cmp - patch12 || exit 1
-diff -u file file
---- file
+diff -u file.orig file
+--- file.orig
 +++ file
 @@ -1,4 +1,4 @@
 -0