No longer need the try-catch fallback; getRootPost will always resolve

getRootPost used to throw an exception when the root post was not fed to the
PostRevision object; now it will already look it up using Collection.
The manual lookup in the catch is now useless.

Change-Id: I3ef5bb164fb9bece0ed2e62454d07d114a670631
Merge "No longer need the try-catch fallback; getRootPost will always resolve"