git-apply: more consistency checks on gitdiff filenames

There's some duplication of filenames when doing filename operations
(creates, deletes, renames and copies), and this makes us verify that
the pathnames match when they should.

apply.c

@@ -188,17 +188,59 @@ static int gitdiff_hdrend(const char *line)
 	return -1;
 }
 
+/*
+ * We're anal about diff header consistency, to make
+ * sure that we don't end up having strange ambiguous
+ * patches floating around.
+ *
+ * As a result, gitdiff_{old|new}name() will check
+ * their names against any previous information, just
+ * to make sure..
+ */
+static char *gitdiff_verify_name(const char *line, int isnull, char *orig_name, const char *oldnew)
+{
+	int len;
+	const char *name;
+
+	if (!orig_name && !isnull)
+		return find_name(line, NULL, 1, 0);
+
+	name = "/dev/null";
+	len = 9;
+	if (orig_name) {
+		name = orig_name;
+		len = strlen(name);
+		if (isnull)
+			die("git-apply: bad git-diff - expected /dev/null, got %s on line %d", name, linenr);
+	}
+
+	if (*name == '/')
+		goto absolute_path;
+
+	for (;;) {
+		char c = *line++;
+		if (c == '\n')
+			break;
+		if (c != '/')
+			continue;
+absolute_path:
+		if (memcmp(line, name, len) || line[len] != '\n')
+			break;
+		return orig_name;
+	}
+	die("git-apply: bad git-diff - inconsistent %s filename on line %d", oldnew, linenr);
+	return NULL;
+}
+
 static int gitdiff_oldname(const char *line)
 {
-	if (!old_name && !is_new)
-		old_name = find_name(line, NULL, 1, 0);
+	old_name = gitdiff_verify_name(line, is_new, old_name, "old");
 	return 0;
 }
 
 static int gitdiff_newname(const char *line)
 {
-	if (!new_name && !is_delete)
-		new_name = find_name(line, NULL, 1, 0);
+	new_name = gitdiff_verify_name(line, is_delete, new_name, "new");
 	return 0;
 }