bash prompt: test dirty index and worktree while on an orphan branch

bash prompt: test dirty index and worktree while on an orphan branch

There is only a single test exercising the dirty state indicator on an
orphan branch, and in that test neither the index nor the worktree are
dirty.

Add two failing tests to check the dirty state indicator while either
the index is dirty or while both the index and the worktree are dirty
on an orphan branch, and to show that the dirtiness of the index is
not displayed in these cases (the fourth combination, i.e. clean index
and dirty worktree are impossible on an orphan branch).  Update the
existing dirty state indicator on clean orphan branch test to match
the style of the two new tests, most importantly to use 'git checkout
--orphan' instead of cd-ing into a repository that just happens to be
empty and clean.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Jeff King <peff@peff.net>