BUGFIX: make `git_clear_vars()` actually clear variables

The current `git_clear_vars()` method appears to have iterated over all the existing set variable correctly, but seemingly never actually
cleared them.  This fixes that behavior as best I can tell.

(That said, the extra variables weren't really hurting anybody before this fix, so maybe another option is the entire function should be deprecated to reduce the amount of operations a `gs` has to do in the interest of speed. Let me know what you prefer!)