Mercurial 4.9 release

This is an overview of 4.9 release.

1. Security Fixes

It was possible to use symlinks and subrepositories to defeat Mercurial's path-checking logic and write files outside a repository. This has been fixed. Users on older versions can either disable subrepositories with

[subrepos]
allowed = false

in their configuration or by ensuring any cloned repositories don't contain malicious symlinks.

2. New Features

• 'hg histedit' will now present a curses UI if curses is available and 'ui.interface' or 'ui.interface.histedit' is set to 'curses'.
• The 'sparse-revlog' delta strategy is enabled by default for new repositories.

  See hg help config.format.sparse-revlog for details.

3. New Experimental Features

4. Other Notable Features

• New rewrite.update-timestamp=True option to update the commit timestamp on history editing (e.g. amend.)

• New ui.message-output=stderr option for scripting, which prevents status messages from interleaved.

• New rootglob: filename pattern for a glob that is rooted at the root of the repository. See hg help patterns and hg help hgignore for details.

• Some more reimplementation of ancestry algorithms in Rust for better performance.

5. Bug Fixes

6. Backwards Compatibility Changes

• Boolean options passed to the logtoprocess extension are now formatted as "0" or "1" instead of "None", "False", or "True".
• The logtoprocess extension no longer supports invalid "ui.log()" arguments. A log message is always formatted and passed in to the "$MSG1" environment variable.

7. Internal API Changes

• Status messages may be sent to a dedicated stream depending on configuration. Don't use "ui.status()", etc. as a shorthand for conditional writes. Use "ui.write()" for data output.
• Add 'exthelper' class to simplify extension writing by allowing functions, commands, and configitems to be registered via annotations. The previous APIs are still available for use.
• The extension hook "extsetup" without a 'ui' argument has been deprecated, and will be removed in the next version. Add a 'ui' argument to avoid the deprecation warning.