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.
%include map-cmdline.default
[templates]
changeset = '{cset}{branches}{bookmarks}{tags}{lphase}{parents}{luser}{ldate}{summary}\n'
changeset_verbose = '{cset}{branches}{bookmarks}{tags}{lphase}{parents}{luser}{ldate}{lfiles}{lfile_copies_switch}{description}\n'