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

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/commitgraph/CommitGraphWriter.java

@@ -222,6 +222,9 @@ private void writeChunks(ProgressMonitor monitor,
 				}
 				chunk.data.get().writeTo(out);
 				break;
+			default:
+				throw new IllegalStateException(
+						"Don't know how to write chunk " + chunkId); //$NON-NLS-1$
 			}
 		}
 	}