git-gui: Allow 'Create New Repository' on existing directories

git-gui: Allow 'Create New Repository' on existing directories

Often users setup a few source files and get a project rolling
before they create a Git repository for it.  In such cases the
core Git tools allow users to initialize a new repository by
simply running `git init` at the desired root level directory.

We need to allow the same situation in git-gui; if the user is
trying to make a new repository we should let them do that to any
location they chose.  If the directory already exists and already
has files contained within it we still should allow the user to
create a repository there.  However we still need to disallow
creating a repository on top of an existing repository.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>