Expose base class for Git source plugins

Add a `_GitSourceBase` class to act as the base class for deriving
source plugins that work with Git. The first user of this base class is
the `git` source plugin that's in core at the moment.

The contents of this base class is almost identical to the existing `GitSource`
class. The two notable differences are:

- the private methods now have leading underscores, as some were missing
  it originally
- the name of the class

Note that we are exposing a private member here as we expect it to move to a
separate package soon. See the following discussion for more details:
#739 (comment 124819869)

Fixes #739.