merge alg needs to check for cyclic dependencies

when creating needs to remove metadata, make a separate commit with no
metadata to allow us to spot what's happening.  should only do this
with option to permit it!  Also need to do this when merging.

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
		topbloke branch has all the deps of the topgit
		 branch (topgit branches mapped into topbloke
		 branches) plus the topgit branch itself named in
 		 flags
	       	reject topbloke imports of topgit branches which
		 also contain topbloke metadata
	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 but for now abstract away commit and
 other ref update

also would like some kind of --ff-only update ?