topic: fix compatibility issues caused because of change in transaction API In 36f08ae87ef687be53a59bd87376bcfbe4479205 in core mercurial, `_validator` attribute to transaction class was removed and a dict was introduced. It added a `addvalidator()` function to transaction class which can be used to register multiple validator callbacks. This updates the code to use `addvalidator()` when `_validator` attribute is not present.

obsexchange: fallback to simpler request for dumb http server If the http server cannot accept many arguments, avoid sending too large requests (at the cost of more data exchanged). A better solution would be to aggregate contiguous range, but this is a question for later.

obsexchange: swap the 'common' and 'missing' clause

obsdiscovery: avoid considering locally hidden changeset This looks like a dubious optimisation

obsdiscovery: add a test where some common node are hidden This is useful to prevent regression.

tests: use appropriate Python (2 or 3) for dummyssh

evolve: add missing 'of' in message The message refers to the changeset that is currently evolved. Therefore the message needs an additional 'of' to be correct.

test: fix graph label in test-push-checkheads-pruned-B5.t They were wrong.

readme: mention compat string format

readme: copywrite the section about branch policy, wrap long lines

compat: update hg-X.Y compat comments and test them

contrib: add a script to check release in compat comments that have node hash

changelog: add an entry about the cleanup

evolve: drop compat for pre-4.5 context.memfilectx()