merge-tools: when calling external merge tool, describe the resolve inputs It is a common complaint that a user will be running some operation (histedit, rebase, evolve, etc.), get into a merge-conflict situation, and not understand what they are seeing - it is possible that the merge tool is configured to display the hash, but it's difficult for most merge tools to display a good snippet of the description. In the worst case, configuring this template will lead to output that is immediately covered by a terminal application, maybe the user can hit ctrl-z to see it. In the common case, the output will be in a terminal window and a GUI program will start, and it should be possible to view both the terminal and the GUI program at the same time. Differential Revision: https://phab.mercurial-scm.org/D5094

templatekw: deprecate p1rev/p2rev/p1node/p2node in favor of p1/p2

templatekw: add p1/p2 keywords which switches the current ctx This is just an example of mappingdict. I have no idea what should be displayed as {p1|json}. Currently it is an empty dict since {'ctx'} isn't displayable, which is clearly useless and should be changed later.

templater: add wrapper for a single template mapping This can be used to nest template mappings without inserting a sequence-like layer. See the next patch for example.

templatekw: extract internal "{rev}:{node|formatnode}" template to constant This will be used later.

test: remove changes preparing the drop of `HGMERGE` These changes are no longer necessary so we better drop them.

run-tests: define the default merge tool through configuration Using the `HGMERGE` environment variable generates confusion as it overrides any value set through configuration. By setting the default value through the default HGRC, the usual overriding rules apply and it should reduce confusion.

run-tests: explicitly declare the list of dropped environment variable It will make the list clearer and more maintainable.

test: drop usage of `HGMERGE` in `test-lfconvert.t` Using configuration instead of environment variable has a strange effect on the output, but no impact on the end result.

test: drop usage of `HGMERGE` in `test-merge-types.t` Using configuration instead of environment variable has a strange effect on the output, but no impact on the end result.