tests: improve test-mv-cp-st-diff.t readability

obsolete: refuse to push divergent changeset As other troubles `unstable` and `bumped`. Followup patches may simplify the push code with unification of "obsolescence troubles" handling.

obsolete: add a divergent method on context The same we have `unstable` and `bumped`. Convenient method to access troubles information in general may land later. This get actual use and testing in the next changesets.

obsolete: add revset and test for divergent changesets This changesets add a new `divergent()` revset similar to `unstable()` and `bumped()` one. Introducting this revset allows actuall test of the divergent detection.

obsolete: detect divergent changesets Divergent changeset are final successors (non obsolete) of a changeset who compete with another set of final successors for this same changeset. For example if you have two obsolescence markers A -> B and A -> C, B and C are both "divergent" because they compete to be the one true successors of A. Public revision can't be divergent. This function is used and tested in the next changeset.

obsolete: drop successors sets which are subset of another one If both "(B,)" and "(B, C)" are successors set of "A", "(B,)" is dropped. We won't be interrested in detection such divergence scenario.

obsolete: compute successors set Successors set are an important part of obsolescence. It is necessary to detect and solve divergence situation. This changeset add a core function to compute them, a debug command to audit them and solid test on the concept. Check function docstring for details about the concept.

mq: don't fail when removing a patch without patch file from series file

largefiles: align rm warnings with warnings used in core

merge with stable