CommitGraphWriter: throw exception on unknown chunk

CommitGraphWriter: throw exception on unknown chunk

CommitGraphWriter first defines the chunks and then writes them. If at
write time a chunk is unknown, it is ignored. This is brittle: if
somebody adds a chunk to the header but not to the actual writing, the
commit-graph is broken and there is no error reported anywhere.

Throw exception if at write time a chunk is unknown. This can only
happen by a coding error in the writer.

Change-Id: Iade677bb6ce368b6941b75a21c622917afa3b751