add more control to indexState() return-value

add more control to indexState() return-value

The indexState() method was enhanced to be more configurable. A bitmask
controls which of the optional parts are reported. All data about
the worktree is not reported anymore by this method which makes the
interface more cleaner for users wanting to test only the state of the
index.
This was done because the previous version reported always so much
additional data that it was hard to write good assertions against it.

Change-Id: I9b481e97f8fcf3fcdbb785b801dc07bfa85dcc33
Signed-off-by: Christian Halstrick <christian.halstrick@sap.com>
Signed-off-by: Stefan Lay <stefan.lay@sap.com>