GITC: Add gitc-init subcommand to repo.

Adds the new gitc-init command to set up a GITC client. Gitc-init
sets up the client directory and calls repo init within it. Once
the repo is initialized, then generates a GITC manifest file
by using git ls-remote on each project and retrieving the HEAD SHA
to use as the revision attribute.

Gitc-init inherits from and has all the options as repo init.

Change-Id: Icd7e47e90eab752a77de7c80ebc98cfe16bf6de3