fix: add suboption for configuring execution order of tools
This could be accomplished by using wrapper scripts, but that would diminish
the usefulness of the incremental formatting logic. Configuring execution order
along with other things in the hgrc is probably more convenient anyway.
This change highlights some awkwardness with suboptions and default values,
which should be addressed separately.
Differential Revision: https://phab.mercurial-scm.org/D5237
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