diff: only compute patches when requested

When we're called with no hunk or line callback, we don't need to
do the hunk or line computation.

src/diff_patch.c

@@ -284,11 +284,12 @@ int git_diff_foreach(
 		if (git_diff_delta__should_skip(&diff->opts, patch.delta))
 			continue;
 
-		if ((error = diff_patch_init_from_diff(&patch, diff, idx)) < 0)
-			break;
-
-		if (!(error = diff_patch_invoke_file_callback(&patch, &xo.output)))
-			error = diff_patch_generate(&patch, &xo.output);
+		if ((error = diff_patch_invoke_file_callback(&patch, &xo.output)) == 0) {
+			if (hunk_cb || data_cb) {
+				if ((error = diff_patch_init_from_diff(&patch, diff, idx)) == 0)
+					error = diff_patch_generate(&patch, &xo.output);
+			}
+		}
 
 		git_patch_free(&patch);