+The client MUST write all obj-ids which it only has shallow copies
+of (meaning that it does not have the parents of a commit) as
+'shallow' lines so that the server is aware of the limitations of
+the client's history. Clients MUST NOT mention an obj-id which
+it does not know exists on the server.
+
+The client now sends the maximum commit history depth it wants for
+this transaction, which is the number of commits it wants from the
+tip of the history, if any, as a 'deepen' line. A depth of 0 is the
+same as not making a depth request. The client does not want to receive
+any commits beyond this depth, nor objects needed only to complete
+those commits. Commits whose parents are not received as a result are
+defined as shallow and marked as such in the server. This information
+is sent back to the client in the next step.
+
+Once all the 'want's and 'shallow's (and optional 'deepen') are
+transferred, clients MUST send a flush-pkt, to tell the server side
+that it is done sending the list.
+
+Otherwise, if the client sent a positive depth request, the server
+will determine which commits will and will not be shallow and
+send this information to the client. If the client did not request
+a positive depth, this step is skipped.