Clean up git client type.
Review Request #10086 — Created July 14, 2018 and submitted
When we detect what type of git checkout we're dealing with (plain git,
git-svn, or git-p4), that would get assigned to a public attribute with a
string type. This change fixes things so it's a private attribute with
Ran unit tests.
We could also use enum34 on python 2.x and use an enum.Enum here.