We could have a "darcs check --hard" command that would check hard (as
in cuadradic time or above) invariants of the current repo, like:

1. no duplicate PatchInfo (Inspired by http://bugs.darcs.net/issue1868)
2. more stuff that is not tested/known to be implemented by current
patch-related code?
2. more properties about hashed files? (no cycle?)

1. (no) duplicate PatchInfo

This should not need not be quadratic, only slightly super-linear:
traverse the complete inventory chain; for each patch calculate the meta
data hash and use it as key to accumulate the patch representation
hash(es) into a HashMap indexed by the meta data hash; finally traverse
the map and display any duplicates found.