Add a class for interacting with git config

This commit adds the GitConfigStore class. This allows us to easily get and set keys and values in git config.

The purpose of this class is to enable commands to use  `git config` in a unified manner. The functions to get and set the default imerge name now use this class.

The class is memoized to avoid repeated calls to `git config`.

Tests pass.
This storage scheme limits the `GitConfigStore` class to single-valued config settings, correct?  That is OK, but it might be worth mentioning this limitation in the class docstring.