push: use stepsdone for obsmarkers push We do not have infrastructure to include obsolescence markers in the bundle2 push from core. But extensions may so we make sure it would not be sent twice.

push: introduce a discovery step for obsmarker The discovery step is still not doing anything smart. But this will allow extension to wrap it.

push: move the list of obsmarker to push into the push operation The list is now carried in the push operation, this will let extensions override it.

push: explicitly encode a list of obsmarkers to push Sending obsmarkers through pushkey requires extra encoding (since pushkey can't take binary content) and slicing (since we can hit http header limit). As we send all obsolescences markers that exists in the repo for each push, we used to just look at the content of the "obsolete" pushkey namespace (already encoded and sliced) and send its content. However, future changeset will make it possible to push only parts of the obsmarkers. To prepare this we now explicitly encode a list of markers. The list of markers is still "all of them" but future changeset will takes care of that. The new code uses a "_protected" method but that seems reasonable to keep it private as this is the is the only external user of it and this whole pushing obsmarker through pushkey things in fairly hacky already)

merge-tools: add a `premerge=keep-merge3` config option This value leaves premerge markers that includes the merge base too. This is a the same as what `internal:merge3` would do.

merge-tools: make premerge valid values extensible We want to introduce a version leaving merge3 style markers.

mergetools: add a test for premerge --keep It works! No surprise.