Add `gitfs_base` config option to set base branch

Add the ability to use a branch other than `master` as the base
environment by setting the `gitfs_base` option in the master config
file.

The Problem:
When using both a feature-branch based git workflow and gitfs, then in order
to test changes in the feature-branch, the top file in the master branch
may need to be modified. It is undesirable to change the master branch in order
to test a feature branch.

Intended Use:
This change is intended to allow for a dev/testing/staging environment
with its own salt master to dynamically change which branch to read the
top file from (with a restart of the salt master). E.g., the test salt
master can set its base branch to `feature` -- which has a modified top
file -- in order to test the changes in `feature` without requiring
changes in `master`.
Merge pull request #6960 from jesusaurus/gitfs/base

Add `gitfs_base` config option to set base branch