Add export_from_git.py to take snapshots from chromium and ninja.

Split out the Submodule class and related functions from
init-repository.py into a separate python module git_submodule.py.
This is necessary in order to reuse the code for the export script.

The export_from_git.py script can be used to export files
from a git repository into an arbitrary directory.
It spiders submodules and applies some pattern matching to remove
unwanted files.
This script can be used to create flattened chromium snapshots
to be used in our CI system.

Change-Id: Iade8126fae6c28b5347c9d6e08941e28d3e0e7be
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>