Add definition for 'quiet' capability.

dulwich/protocol.py

@@ -52,6 +52,7 @@ CAPABILITY_MULTI_ACK_DETAILED = b'multi_ack_detailed'
 CAPABILITY_NO_DONE = b'no-done'
 CAPABILITY_NO_PROGRESS = b'no-progress'
 CAPABILITY_OFS_DELTA = b'ofs-delta'
+CAPABILITY_QUIET = b'quiet'
 CAPABILITY_REPORT_STATUS = b'report-status'
 CAPABILITY_SHALLOW = b'shallow'
 CAPABILITY_SIDE_BAND_64K = b'side-band-64k'