help: assigning categories to existing commands
I'm separating this into its own commit so people can bikeshed over the actual
categorization (vs the support for categories). These categories are based on
the help implementation we've been using internally at Google, and have had
zero complaints.
Differential Revision: https://phab.mercurial-scm.org/D5067
#require docutils gettext
$ $TESTDIR/check-gendoc it
checking for parse errors