Recognize submodules even when --no-git/use_git=False

In [this comment](https://github.com/astropy/astropy-helpers/issues/40#issuecomment-45519368) I mentioned that when use of git is disabled, an astropy_helpers submodule is no longer recognized as a submodule.  The auto-upgrade feature of ah_bootstrap is not meant to be used with astropy_helpers is available as a submodule (i.e. is in a developer checkout), but this was being violated when use of git is disabled entirely.

With this patch `--no-git` still disables execution of `git` in a subprocess, but instead simply reads the `.gitmodules` file, if present, to determine if `astropy_helpers` is a git submodule.