patch_parse: handle absence of "index" header for new/deleted cases

This follows up on 11de594 which added
support for parsing patches without extended headers (the "index
<hash>..<hash> <mode>" line); issue libgit2#5267.

We now allow transition from "file mode" state to "path" state directly
if there is no "index", which will happen for patches adding or deleting
files as demonstrated in added test case.

Author: dlax

src/patch_parse.c

@@ -407,6 +407,7 @@ static const parse_header_transition transitions[] = {
 
 	{ "--- "                , STATE_DIFF,       STATE_PATH,       parse_header_git_oldpath },
 	{ "--- "                , STATE_INDEX,      STATE_PATH,       parse_header_git_oldpath },
+	{ "--- "                , STATE_FILEMODE,   STATE_PATH,       parse_header_git_oldpath },
 	{ "+++ "                , STATE_PATH,       STATE_END,        parse_header_git_newpath },
 	{ "GIT binary patch"    , STATE_INDEX,      STATE_END,        NULL },
 	{ "Binary files "       , STATE_INDEX,      STATE_END,        NULL },

tests/diff/parse.c

@@ -107,6 +107,29 @@ void test_diff_parse__no_extended_headers(void)
 	git_diff_free(diff);
 }
 
+void test_diff_parse__add_delete_no_index(void)
+{
+	const char *content =
+	    "diff --git a/file.txt b/file.txt\n"
+	    "new file mode 100644\n"
+	    "--- /dev/null\n"
+	    "+++ b/file.txt\n"
+	    "@@ -0,0 +1,2 @@\n"
+	    "+one\n"
+	    "+two\n"
+	    "diff --git a/otherfile.txt b/otherfile.txt\n"
+	    "deleted file mode 100644\n"
+	    "--- a/otherfile.txt\n"
+	    "+++ /dev/null\n"
+	    "@@ -1,1 +0,0 @@\n"
+	    "-three\n";
+	git_diff *diff;
+
+	cl_git_pass(git_diff_from_buffer(
+		&diff, content, strlen(content)));
+	git_diff_free(diff);
+}
+
 void test_diff_parse__invalid_patches_fails(void)
 {
 	test_parse_invalid_diff(PATCH_CORRUPT_MISSING_NEW_FILE);