Perf dashboard shouldn't show the full git hash
https://bugs.webkit.org/show_bug.cgi?id=135083

Reviewed by Benjamin Poulain.

Detect Git/Mercurial hash by checking the length.

If it's a hash, use the first 8 characters in the label
while retaining the full length to be used in hyperlinks.

* public/js/helper-classes.js:
(.this.formattedRevisions):
(TestBuild):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@171258 268f45cc-cd09-0410-ab3c-d52691b4dbfc