When an extension wants to abandon more than one changes in the same
project, batch them together inside a BatchUpdate will be much better
than calling Abandon REST API multiple times, so this change provides
the batch mode.
Also changed AbandonUtil to group changes by project and use the new
batchAbandon code.
Change-Id: I9d89b58d4cfa469b666a234af1a564c1f5211b19