We add a number of tests, conditional on the Git version, to
validate the functionality of the --worktree option when
installing or uninstalling.
Note that because this simply passed through to the "git config"
command, we inherit the behaviour that --worktree functions
like --local when only a single, default working tree is in use.
And if we have a Git version older than 2.20.0, we confirm that
the --worktree option is not supported.
