Mercurial 3.9 release

Features

ui.textwidth can now be set to define width of help text
separate() template function added
ui.rollback can be set to false to disable the hg rollback command
fail-<command> hooks now run when a command fails
experimental.graphstyle.* config options to control styling of graphs in console
experimental.histedit.autoverb allows histedit lines beginning with "verb!" to be interpreted as histedit actions
[hostsecurity] config section for defining advanced per-host security settings
ability to define the SHA-256 and SHA-512 hashes of pinned server certificates
ability to define CA certificates on a per-host basis
ability to define the minimum TLS protocol version on a global or per-host basis
sort() revset can now perform topological sorts using the topo option
hgweb can now render JSON for filelog, filerevision, summary, and search web commands
[paths] entries can now define a pushrev sub-option to control which revisions to push by default
The experimental 'journal' extension was added, allowing users to view the previous positions of bookmarks and the working copy

This release includes many improvements, including (but not limited to):

performance of hg diff has been improved
chg now detects more changes to the configuration and execution environment
SSL/TLS code has been significantly refactored and now is consistent across all consumers (HTTPS, SMTPS)
performance improvements to server communication (particularly for the largefiles and remotefilelog extensions)
connections to servers whose certificate authority (CA) is unknown are now refused even if no CA certificates are available
(see SecureConnections for details)
fingerprints of server certificates are now printed using SHA-256 instead of SHA-1
reads and writes to certain files is now robust and avoids more race conditions and edge cases (see ExactCacheValidationPlan)
performance improvements to certain revsets
cloning will no longer prompt for a password multiple times when cloning from a server that requires a password
annotate view in hgweb now groups lines into blocks depending on their revision and highlights lines for the current revision
hgweb now displays extra information and also navigation links in popups for each line in annotate view

A full overview is available on WhatsNew.