Resolve crash with diff against empty file

It is not legal inside our `p_mmap` function to mmap a zero length
file.  This adds a test that exercises that case inside diff and
fixes the code path where we would try to do that.

The fix turns out not to be a lot of code since our default file
content is already initialized to "" which works in this case.

Fixes #1210

src/diff_output.c

@@ -376,6 +376,9 @@ static int get_workdir_content(
 			goto close_and_cleanup;
 
 		if (error == 0) { /* note: git_filters_load returns filter count */
+			if (!file->size)
+				goto close_and_cleanup;
+
 			error = git_futils_mmap_ro(map, fd, 0, (size_t)file->size);
 			file->flags |= GIT_DIFF_FILE_UNMAP_DATA;
 		} else {

src/fileops.c

@@ -121,7 +121,7 @@ mode_t git_futils_canonical_mode(mode_t raw_mode)
 
 int git_futils_readbuffer_fd(git_buf *buf, git_file fd, size_t len)
 {
-	ssize_t read_size;
+	ssize_t read_size = 0;
 
 	git_buf_clear(buf);
 

tests-clar/diff/workdir.c

@@ -881,3 +881,46 @@ void test_diff_workdir__checks_options_version(void)
 	err = giterr_last();
 	cl_assert_equal_i(GITERR_INVALID, err->klass);
 }
+
+void test_diff_workdir__can_diff_empty_file(void)
+{
+	git_diff_list *diff;
+	git_tree *tree;
+	git_diff_options opts = GIT_DIFF_OPTIONS_INIT;
+	struct stat st;
+	git_diff_patch *patch;
+
+	g_repo = cl_git_sandbox_init("attr_index");
+
+	tree = resolve_commit_oid_to_tree(g_repo, "3812cfef3661"); /* HEAD */
+
+	/* baseline - make sure there are no outstanding diffs */
+
+	cl_git_pass(git_diff_tree_to_workdir(&diff, g_repo, tree, &opts));
+	cl_assert_equal_i(2, (int)git_diff_num_deltas(diff));
+	git_diff_list_free(diff);
+
+	/* empty contents of file */
+
+	cl_git_rewritefile("attr_index/README.txt", "");
+	cl_git_pass(git_path_lstat("attr_index/README.txt", &st));
+	cl_assert_equal_i(0, (int)st.st_size);
+
+	cl_git_pass(git_diff_tree_to_workdir(&diff, g_repo, tree, &opts));
+	cl_assert_equal_i(3, (int)git_diff_num_deltas(diff));
+	/* diffs are: .gitattributes, README.txt, sub/sub/.gitattributes */
+	cl_git_pass(git_diff_get_patch(&patch, NULL, diff, 1));
+	git_diff_patch_free(patch);
+	git_diff_list_free(diff);
+
+	/* remove a file altogether */
+
+	cl_git_pass(p_unlink("attr_index/README.txt"));
+	cl_assert(!git_path_exists("attr_index/README.txt"));
+
+	cl_git_pass(git_diff_tree_to_workdir(&diff, g_repo, tree, &opts));
+	cl_assert_equal_i(3, (int)git_diff_num_deltas(diff));
+	cl_git_pass(git_diff_get_patch(&patch, NULL, diff, 1));
+	git_diff_patch_free(patch);
+	git_diff_list_free(diff);
+}