Implement 'repo update'

Implement 'repo update'

Implement Repo.Branches() with a pattern.  Add code to handle empty
output and 'stray' values.

Implement MainRepoUpdate, which will call 'git fetch' and 'git merge'
on master and stable-* branches.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>