Expose ReceiveCommand.updateType to check for UPDATE_NONFASTFORWARD

Expose ReceiveCommand.updateType to check for UPDATE_NONFASTFORWARD

When a command's type is UPDATE, JGit might not yet be sure if it
is a fast-forward or not. Expose a utility method to compute the
exact type by performing the merge base test, allowing the type
to be switched to UPDATE_NONFASTFORWARD if old ObjectId is not
contained in new ObjectId.

BaseReceivePack already does this test when validating the incoming
command list, so provide a package level backdoor to set the type
and avoid needing to redo the merge test later.

Change-Id: If5a6fcc50dc4d6f96e9bb0bb7bba15ebe8b86377