index: test dirty index bit

Test that any changes to the index will mark the index as dirty.  Also
ensure that when we initialize a new index, read the index contents
from disk, or write the index contents to disk that we reset the dirty
flag to zero.  Further ensure that an unforced read with dirty contents
(when the on-disk index has not changed) does _not_ reset the dirty
flag as we have not updated the contents of our index and our unsaved
contents remain intact.

tests/index/tests.c

@@ -331,6 +331,59 @@ void test_index_tests__add_frombuffer(void)
 	git_repository_free(repo);
 }
 
+void test_index_tests__dirty_and_clean(void)
+{
+	git_repository *repo;
+	git_index *index;
+	git_index_entry entry = {{0}};
+
+	/* Index is not dirty after opening */
+	cl_git_pass(git_repository_init(&repo, "./myrepo", 0));
+	cl_git_pass(git_repository_index(&index, repo));
+
+	cl_assert(git_index_entrycount(index) == 0);
+	cl_assert(!git_index_is_dirty(index));
+
+	/* Index is dirty after adding an entry */
+	entry.mode = GIT_FILEMODE_BLOB;
+	entry.path = "test.txt";
+	cl_git_pass(git_index_add_frombuffer(index, &entry, "Hi.\n", 4));
+	cl_assert(git_index_entrycount(index) == 1);
+	cl_assert(git_index_is_dirty(index));
+
+	/* Index is not dirty after write */
+	cl_git_pass(git_index_write(index));
+	cl_assert(!git_index_is_dirty(index));
+
+	/* Index is dirty after removing an entry */
+	cl_git_pass(git_index_remove_bypath(index, "test.txt"));
+	cl_assert(git_index_entrycount(index) == 0);
+	cl_assert(git_index_is_dirty(index));
+
+	/* Index is not dirty after write */
+	cl_git_pass(git_index_write(index));
+	cl_assert(!git_index_is_dirty(index));
+
+	/* Index remains not dirty after read */
+	cl_git_pass(git_index_read(index, 0));
+	cl_assert(!git_index_is_dirty(index));
+
+	/* Index is dirty when we do an unforced read with dirty content */
+	cl_git_pass(git_index_add_frombuffer(index, &entry, "Hi.\n", 4));
+	cl_assert(git_index_entrycount(index) == 1);
+	cl_assert(git_index_is_dirty(index));
+
+	cl_git_pass(git_index_read(index, 0));
+	cl_assert(git_index_is_dirty(index));
+
+	/* Index is clean when we force a read with dirty content */
+	cl_git_pass(git_index_read(index, 1));
+	cl_assert(!git_index_is_dirty(index));
+
+	git_index_free(index);
+	git_repository_free(repo);
+}
+
 void test_index_tests__add_frombuffer_reset_entry(void)
 {
 	git_index *index;