Merge MVP script into cbuildbot.

This is done by adding two new configuration options:

git_url -- git repository URL for our manifests.
manifest_version -- Per-build manifest to be stored in GIT

The first defaults to the external git url, and is used for fetching code most of the time. The second, if not None is the git URL for fetching/storing detailed manifest files and the associated build status.

The new file manifest_version.py is a copy (plus edits) of mvp.py from crostools. It's mostly unchanged, except for some minimalization of argument lists and the addition of a 'debug' concept, which means to not push changes to git repositories. This debug concept is controlled by the cbuildbot --debug command line option.

A few new unit tests were added, but not enough over all.

BUG=chromium-os:12467
TEST=

Review URL: http://codereview.chromium.org/6691047

Change-Id: Ib7f1f7e0d39caf6559df9398d0f993eaf4db8a95
diff --git a/lib/cros_build_lib.py b/lib/cros_build_lib.py
index 8728fe3..f7ea5e9 100644
--- a/lib/cros_build_lib.py
+++ b/lib/cros_build_lib.py
@@ -95,7 +95,10 @@
 
   # Print out the command before running.
   if print_cmd:
-    Info('RunCommand: %s' % cmd_str)
+    if cwd:
+      Info('RunCommand: %s in %s' % (cmd_str, cwd))
+    else:
+      Info('RunCommand: %s' % cmd_str)
   cmd_result.cmd = cmd
 
   try: