localrepo: reuse commit of parent filectx entries without rehashing It is currently only amend and debugbuilddag that will pass a filectx to localrepo._filecommit. Amend will usually not hit the case where a the filectx is a parent that just can be reused. Future convert changes will use it more.

obsolete: avoid infinite loop from obs-cycle in divergence (issue4126) As for other currently in place cycle detection, arbitrarily cut the first obsolescence link that create a cycle avoiding the infinite loop. This will have to be made more deterministic in the future but we do not really care right now.

tests: avoid deprecation warning

walkchangerevs: make followfilter a top-level class The class only depends on the 'repo' variable in the closure, so let's move the class out of the function and make it explicit that that (the repo) is all it needs.