Add a new `--force-clean-deps` flag to automate-git.py that deletes/re-downloads all third-party git repos (issue #1348). Should be used in combination with the `--force-clean` flag.

git-svn-id: http://chromiumembedded.googlecode.com/svn/trunk@1787 5089003a-bbd8-11dd-ad1f-f1f9622dbc98
Update automate-git.py to explicitly fetch tags required for release branch builds.

git-svn-id: http://chromiumembedded.googlecode.com/svn/trunk@1791 5089003a-bbd8-11dd-ad1f-f1f9622dbc98