improve "No tests found..." message

Summary:Relates to jestjs#856

ps. I've already signed the CLA.
Closes jestjs#863

Differential Revision: D3137918

fb-gh-sync-id: e58e04b4cc8c5218cc92c9ce8cf0af97b32172f7
fbshipit-source-id: e58e04b4cc8c5218cc92c9ce8cf0af97b32172f7

Author: andresilveirah

src/jest.js

@@ -72,6 +72,7 @@ function buildTestPathPatternInfo(argv) {
   if (argv.onlyChanged) {
     return {
       onlyChanged: true,
+      watch: argv.watch !== undefined,
     };
   }
   if (argv.testPathPattern) {
@@ -97,7 +98,12 @@ function buildTestPathPatternInfo(argv) {
 
 function getNoTestsFoundMessage(patternInfo) {
   if (patternInfo.onlyChanged) {
-    return 'No tests found related to changed and uncommitted files.';
+    const guide = patternInfo.watch
+      ? 'starting Jest with `jest --watch=all`'
+      : 'running Jest without `-o`';
+    return 'No tests found related to changed and uncommitted files.\n' +
+    'Note: If you are using dynamic `require`-calls or no tests related ' +
+    'to your changed files can be found, consider ' + guide + '.';
   }
   const pattern = patternInfo.pattern;
   const input = patternInfo.input;