Support for private git submodules

Private Submodules

With this release, building an app (through Console) with private submodules is now supported.

Build output that was triggered by pushing to a GitHub repo:

convox builds info BRCOWEZXEMP
RUNNING: git clone --progress https://github.com/convox/fake-public-repo.git src
RUNNING: git submodule update --init --recursive
Submodule 'fake-private-repo' (https://github.com/convox/fake-private-repo.git) registered for path 'fake-private-repo'
Submodule path 'fake-private-repo': checked out 'dd41c4403cddb598490cbabc31ddd512556ba1ff'
RUNNING: git checkout b9d76b1ced8f7fa80db19ed3840c33448891cda1
RUNNING: git submodule update --recursive
RUNNING: /usr/local/bin/git-restore-mtime .
RUNNING: docker pull httpd
Using default tag: latest
latest: Pulling from library/httpd
51f5c6a04d83: Pulling fs layer
a3ed95caeb02: Pulling fs layer

Also as a security measure, we no longer log the secret tokens during the build process.