----
advertised-refs = (no-refs / list-of-refs)
+ *shallow
flush-pkt
no-refs = PKT-LINE(zero-id SP "capabilities^{}"
other-tip = obj-id SP refname LF
other-peeled = obj-id SP refname "^{}" LF
+ shallow = PKT-LINE("shallow" SP obj-id)
+
capability-list = capability *(SP capability)
capability = 1*(LC_ALPHA / DIGIT / "-" / "_")
LC_ALPHA = %x61-7A