rhg: use default configitem in `cat`
Now that we've declared the items, we can get rid of the manual fallback.
Mercurial evolves over time, some features, options, commands may be replaced by
better and more secure alternatives. This topic will help you migrating your
existing usage and/or configuration to newer features.
Commands
========
The following commands are still available but their use are not recommended:
``locate``
This command has been replaced by `hg files`.
``parents``
This command can be replaced by `hg summary` or `hg log` with appropriate
revsets. See `hg help revsets` for more information.
``tip``
The recommended alternative is `hg heads`.
Options
=======
``web.allowpull``
Renamed to `allow-pull`.
``web.allow_push``
Renamed to `allow-push`.