merge alg needs to check for cyclic dependencies

wibblings:
	what about strangeness in ancient history wrt merge alg
	topgit import
	       topbloke {base,tip} is ff desc of topgit {base,tip}
	       	with metadata adjusted
	need patch name in metadata
	patch dep metadata syntax unsatisfactory: too twiddly
	flags metadata unsatisfactory: wants to be valued properties

	"checkout" shouldn't do any updates but also want
	"goto" or "push" or "pop" or something which does do updates
	       (at least, the relevant ones)

	topbloke series file under .git records current leaf

	create creates leaf and thus series ?
	insert patch operation to create new patch under this one
	append patch operation to create new patch above this one
	       which is inserted into all clients' deps

what about ops which update multiple refs - atomicity ?
 think about this later