tests/testlib/ext-sidedata-4.py

fix: highlight the required configuration and behavior of the fixer tools

The problem is that `hg help fix` didn't mention *how* to configure the tools,
and while I knew that `{rootpath}` existed in the configuration, I missed that
the tools require reading content from stdin.  (I configured `gofmt` to use
`{rootpath}`, and that had the effect of squashing all changes in a file at `.`
into the first commit and emptying that content from its descendants.)

Basically all this does is put a pointer in the default (command level) help to
the extension level help that mentions the configuration, and moves the
extension level help that documents reading from stdin and writing to stdout to
the top to give it more prominence.  The last sentence is adjusted a bit to
reflect the new location.

# coding: utf8
# ext-sidedata-4.py - small extension to test (differently still) the sidedata
# logic
#
# Simulates a server for a complex sidedata exchange.
#
# Copyright 2021 Raphaël Gomès <[e-mail]>
#
# This software may be used and distributed according to the terms of the
# GNU General Public License version 2 or any later version.


from mercurial.revlogutils import sidedata


def reposetup(ui, repo):
    repo.register_wanted_sidedata(sidedata.SD_TEST2)
    repo.register_wanted_sidedata(sidedata.SD_TEST3)