Fix edge case: app directory is created before first app push

When an app directory (i.e. `/home/dokku/myapp`) is created before the
app is pushed for the first time (which may occur if config files such
as `ENV` are added before the initial push), then the app directory will
not be initialized to a bare git repo nor will the pre-receive hook be
installed because `plugins/git/commands` assumes that if `$APP_PATH`
exists, then the directory must also be git initialized too. This fix
checks for `$APP_PATH/refs` instead.
Merge pull request #391 from mikexstudios/receive-predir-fix

Fix edge case: app directory is created before first app push
Submodules support
Added submodules test
Merge pull request #370 from jazzzz/submodules

Submodule support
Docker build no longer works with github url

Change default URL for progrium/buildstep to include full git clone url
Merge pull request #405 from AlJohri/master

Fix docker build for docker v0.7.3
Add $DOCKER_VERSION to specify docker version
Suppress git advice related to detached HEAD
Merge pull request #400 from aegypius/suppress-git-output-unless-dokku-trace-is-enabled

Suppress git "detached HEAD" notice output
Merge pull request #407 from banyan/specify-docker-version

Add $DOCKER_VERSION to specify docker version