diff: add a test for printing just the header

This was a regression leading up to 1.8.

tests/libgit2/diff/header.c

@@ -0,0 +1,69 @@
+#include "clar_libgit2.h"
+#include "git2/sys/repository.h"
+
+#include "diff_helpers.h"
+#include "diff.h"
+#include "repository.h"
+
+static git_repository *g_repo = NULL;
+
+void test_diff_header__initialize(void)
+{
+}
+
+void test_diff_header__cleanup(void)
+{
+	cl_git_sandbox_cleanup();
+}
+
+#define EXPECTED_HEADER "diff --git a/subdir.txt b/subdir.txt\n"	\
+	"deleted file mode 100644\n"					\
+	"index e8ee89e..0000000\n"					\
+	"--- a/subdir.txt\n"						\
+	"+++ /dev/null\n"
+
+static int check_header_cb(
+	const git_diff_delta *delta,
+	const git_diff_hunk *hunk,
+	const git_diff_line *line,
+	void *payload)
+{
+	int *counter = (int *) payload;
+
+	GIT_UNUSED(delta);
+
+	switch (line->origin) {
+	case GIT_DIFF_LINE_FILE_HDR:
+		cl_assert(hunk == NULL);
+		(*counter)++;
+		break;
+	default:
+		/* unexpected code path */
+		return -1;
+	}
+
+	return 0;
+}
+
+void test_diff_header__can_print_just_headers(void)
+{
+	const char *one_sha = "26a125e";
+	git_tree *one;
+	git_diff *diff;
+	int counter = 0;
+
+	g_repo = cl_git_sandbox_init("status");
+
+	one = resolve_commit_oid_to_tree(g_repo, one_sha);
+
+	cl_git_pass(git_diff_tree_to_index(&diff, g_repo, one, NULL, NULL));
+
+	cl_git_pass(git_diff_print(
+			    diff, GIT_DIFF_FORMAT_PATCH_HEADER, check_header_cb, &counter));
+
+	cl_assert_equal_i(8, counter);
+
+	git_diff_free(diff);
+
+	git_tree_free(one);
+}