mercurial/help/extensions.txt

flags: allow specifying --no-boolean-flag on the command line (BC)

This makes it much easier to enable some anti-foot-shooting features
(like update --check) by default, because now all boolean flags can be
explicitly disabled on the command line without having to use HGPLAIN
or similar. Flags which don't deserve this treatment can be removed
from consideration by adding them to the nevernegate set in fancyopts.

This doesn't make it any easier to identify when a flag is set: opts
still always gets filled in, either with the user-specified flag value
or with the default from the flags list in the command
table. Improving that would probably clean things up a bit, but for
now if you want a boolean flag and care if it was explicitly false or
default false (or true, but nobody uses that functionality because
before now it was nonsense) you need to use None as your default
rather than True or False.

This doesn't (yet) update help output, because I'm not quite sure how
to do that cleanly.

Mercurial has the ability to add new features through the use of
extensions. Extensions may add new commands, add options to
existing commands, change the default behavior of commands, or
implement hooks.

To enable the "foo" extension, either shipped with Mercurial or in the
Python search path, create an entry for it in your configuration file,
like this::

  [extensions]
  foo =

You may also specify the full path to an extension::

  [extensions]
  myfeature = ~/.hgext/myfeature.py

See :hg:`help config` for more information on configuration files.

Extensions are not loaded by default for a variety of reasons:
they can increase startup overhead; they may be meant for advanced
usage only; they may provide potentially dangerous abilities (such
as letting you destroy or modify history); they might not be ready
for prime time; or they may alter some usual behaviors of stock
Mercurial. It is thus up to the user to activate extensions as
needed.

To explicitly disable an extension enabled in a configuration file of
broader scope, prepend its path with !::

  [extensions]
  # disabling extension bar residing in /path/to/extension/bar.py
  bar = !/path/to/extension/bar.py
  # ditto, but no path was supplied for extension baz
  baz = !