* Test only for the presence of a .git directory in virtualenvwrapper

Instead of using both $(git rev-parse --show-toplevel) and a check for
a .git directory, use just the latter. As well as being redundant
the former does not work quite so well when using multiple worktrees;
each worktree will be treated as a separate project.

* Unset ENV_NAME & deactivate if no virtualenv found

This addresses #4603 without breaking current behaviour (where current
behaviour is correct).

When changing directories, if there is no environment matching
ENV_NAME, ENV_NAME is emptied and deactivate called if there is a
current environment active (based on CD_VIRTUAL_ENV).

* Use path comparison not string comparison for paths

This will solve part of issue #4255 where WORKON_HOME is defined with a
trailing slash or not normalised in some way, as well as instances
where symlinks are used, and any other instances where constructed
paths don't exactly match even though they go to the same file.
Co-authored-by: NRobby Russell <robby@planetargon.com>