magit-init: sanitize

Clearly separate the interactive and the non-interactive case by moving
all the questions this function might ask inside `interactive'.  This
function isn't intended for non-interactive use, but when it is used
that way, then the directory is now always (re-)initialized - no
questions asked.

Interactively, when the chosen directory doesn't exist yet, then don't
ask the user whether she really wants to create that directory.  She
just selected that directory and creating a directory is usually part
of creating a repository there.  Asking again is just annoying.
magit-test.el: remove magit-init test

Now that `magit-init' simply calls `git init DIRECTORY' when called
non-interactively, this test is rather pointless.