Issue #9932: GitPillar.update called on 1st branch

GitPillar.update was only called for first entry
in ext_pillar, which usually is the master branch.
Issue #9932: Update git pillar after init

Update GitPillar by calling update() after initializing the repo.
Also, pass the branch to Pillar class constructor.
Fixed issue #9932

If an ext_pillar item is not loaded and its branch
is not present in pillar_roots, the corresponding
salt environment will not be available. However,
this is not an issue, since it will be available
when all of ext_pillar items are parsed; therefore,
when trying to get the SLS from that non-present
salt environment, we should merely log a debug
message and continue on.
Merge pull request #9992 from pakdel/issue-9932

Fixed Issue #9932: Only the first git repository is initialized and working using git_pillar