Groups-dev distbuild checked wrong Git repository commit hash

The distbuild process checked .git/packed-refs instead of
.git/refs/heads/master. This file is required for triggering
automatic staging site rebuild.

Change-Id: I177dda27140ac6af7f3c94c9b161b5b24c4ba0c0
Document basic admin hints for jeepyb.

Somewhat obvious, but would have saved me a debugging round-trip.

Change-Id: I31a69b8ba0413ed09423822b817f0d05517751b0
Explain API projects a little.

I was thoroughly confused, and suspect others without direct
experience may be too.

Change-Id: I9003eeaefc7734ef360ff893cec8c828fd51f254
Merge "Groups-dev distbuild checked wrong Git repository commit hash"
Merge "Document basic admin hints for jeepyb."
Merge "Explain API projects a little."