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.

test-merge-tools: introduce a "revision 4" that merges with conflict We need conflicts to test the premerge=keep configuration.

merge: add an internal:merge3 tool This variant gives access to a feature already present in ``internal:merge``: displaying merge base content. In the basic merge (calling ``hg merge``) case, including more context to the merge markers is an interesting addition. But this extra information is the only viable option in case conflict from grafting (, rebase, etc…). When grafting ``source`` on ``destination``, the parent of ``source`` is used as the ``base``. When all three changesets add content in the same location, the marker for ``source`` will contains both ``base`` and ``source`` content. Without the content of base exposed, there is no way for the user to discriminate content coming from ``base`` and content commit from ``source``. Practical example (all addition are in the same place): * ``destination`` adds ``Dest-Content`` * ``base`` adds ``Base-Content`` * ``source`` adds ``Src-Content`` Grafting ``source`` on ``destination`` will produce the following conflict: <<<<<<< destination Dest-Content ======= Base-Content Src-Content >>>>>>> source This that case there is no way to distinct ``base`` from ``source``. As a result content from ``base`` are likely to slip in the resolution result. However, adding the base make the situation very clear: <<<<<<< destination Dest-Content ||||||| base Base-Content ======= base Base-Content Src-Content >>>>>>> source Once the base is added, the addition from the grafted changeset is made clear. User can compare the content from ``base`` and ``source`` to make an enlightened decision during merge resolution.

internal:merge: update documentation Highlight the fact there are two regions in the markers and what their contents are. This prepares for the arrival of merge3.