test: fix commit message test for branches (angular#5004)

For travis runs on branches, local master never exists there's no branch
tracking origin/master.

This causes `git fetch origin master` to fetch into `FETCH_HEAD` instead
of master.

This PR will explicitely fetch origin master into local master, and use
that for the log comparison.

Author: filipesilva

scripts/test-commit-messages.js

@@ -32,7 +32,7 @@ logger
 // Note: This is based on the gulp task found in the angular/angular repository
 
 exec(
-  'git fetch origin master && git log --reverse --format=%s origin/master.. --no-merges',
+  'git fetch origin master:master && git log --reverse --format=%s master.. --no-merges',
   (error, stdout, stderr) => {
     if (error) {
       logger.fatal(stderr);