Don't use API exception in RebaseTodoLine

Don't use API exception in RebaseTodoLine

This came up while testing the proposed buck build for jgit. With buck
we can introduce smaller modules to allow for more concurrency during
build and to better control inner structure of jgit. Trying to put the
porcelain API into a different module than lower level implementation
classes failed since RebaseTodoLine used a porcelain API exception
causing a dependency cycle on the proposed modules. Using an exception
defined on the same abstraction level fixes this problem.

Change-Id: I26a5353e1a8fc23e67d8ce61309bd964f7665bcb
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>
Signed-off-by: Christian Halstrick <christian.halstrick@sap.com>