Use the version tag when checking out chromium

Instead of grepping for the VERSION file update in the git log
check out the sha1 that has been tagged with the requested version.
Also harden the regexp we use to translate svn revisions to git
sha1's and further clean up the findShaAndCheckout function.

Change-Id: Iaeb7ebee558b7ebadaf3428e06626736601198cf
Reviewed-by: Michael Bruning <michael.bruning@digia.com>