Remove some monkey patching used to allow git-upstream to work with
GitPython releases older than 0.3.2, as the project requirements now
depends on 1.0.1 or newer.
GitPython release from 2.0.9 onwards removes the '__slots__'
definition. This allowed for the git_dir attribute existence to be
checked to determine whether to patch the Commit class imported from
the GitPython project to add this attribute for older versions of the
API.
Change-Id: I3d83785b16cd0650942c7112b5cafffc21dddba7
Closes-Bug: #1634053
5c0a4a7e4d