Move incremental_build.sh to run-on-changed-packages (#3846)

Switch incremental_build.sh from using the older check_changed_packages
implemented in bash to the newer (tested/testable) Dart implementation
via --run-onchanged-packages.

Also clarifies in help that the flag runs on all packages when nothing
has changed.

script/tool/lib/src/common.dart

@@ -203,6 +203,7 @@ abstract class PluginCommand extends Command<void> {
     argParser.addFlag(_runOnChangedPackagesArg,
         help: 'Run the command on changed packages/plugins.\n'
             'If the $_pluginsArg is specified, this flag is ignored.\n'
+            'If no plugins have changed, the command runs on all plugins.\n'
             'The packages excluded with $_excludeArg is also excluded even if changed.\n'
             'See $_kBaseSha if a custom base is needed to determine the diff.');
     argParser.addOption(_kBaseSha,