index: test that we round-trip nsecs

Test that nanoseconds are round-tripped correctly when we read
an index file that contains them.  We should, however, ignore them
because we don't understand them, and any new entries in the index
should contain a `0` nsecs field, while existing preserving entries.

tests/index/nsec.c

@@ -0,0 +1,78 @@
+#include "clar_libgit2.h"
+#include "index.h"
+#include "git2/sys/index.h"
+#include "git2/repository.h"
+#include "../reset/reset_helpers.h"
+
+static git_repository *repo;
+static git_index *repo_index;
+
+#define TEST_REPO_PATH "nsecs"
+
+// Fixture setup and teardown
+void test_index_nsec__initialize(void)
+{
+	repo = cl_git_sandbox_init("nsecs");
+	git_repository_index(&repo_index, repo);
+}
+
+void test_index_nsec__cleanup(void)
+{
+	git_index_free(repo_index);
+	repo_index = NULL;
+
+	cl_git_sandbox_cleanup();
+}
+
+static bool has_nsecs(void)
+{
+	const git_index_entry *entry;
+	size_t i;
+	bool has_nsecs = false;
+
+	for (i = 0; i < git_index_entrycount(repo_index); i++) {
+		entry = git_index_get_byindex(repo_index, i);
+
+		if (entry->ctime.nanoseconds || entry->mtime.nanoseconds) {
+			has_nsecs = true;
+			break;
+		}
+	}
+
+	return has_nsecs;
+}
+
+void test_index_nsec__has_nanos(void)
+{
+	cl_assert_equal_b(true, has_nsecs());
+}
+
+void test_index_nsec__staging_maintains_other_nanos(void)
+{
+	const git_index_entry *entry;
+
+	cl_git_rewritefile("nsecs/a.txt", "This is file A");
+	cl_git_pass(git_index_add_bypath(repo_index, "a.txt"));
+	cl_git_pass(git_index_write(repo_index));
+
+	cl_git_pass(git_index_write(repo_index));
+
+	git_index_read(repo_index, 1);
+	cl_assert_equal_b(true, has_nsecs());
+
+	cl_assert((entry = git_index_get_bypath(repo_index, "a.txt", 0)));
+	cl_assert_equal_i(0, entry->ctime.nanoseconds);
+	cl_assert_equal_i(0, entry->mtime.nanoseconds);
+}
+
+void test_index_nsec__status_doesnt_clear_nsecs(void)
+{
+	git_status_list *statuslist;
+
+	cl_git_pass(git_status_list_new(&statuslist, repo, NULL));
+
+	git_index_read(repo_index, 1);
+	cl_assert_equal_b(true, has_nsecs());
+
+	git_status_list_free(statuslist);
+}