513498: manage release and snapshot repositories in git

* This enables updating of composites independently of Mylyn builds,
e.g. to pick up Docs releases.
* Generate p2.index files.

Change-Id: I883d818e623053ae41cdf3721479f4f522b91e24
Task-Url: https://bugs.eclipse.org/bugs/show_bug.cgi?id=513498
59 files changed