The general regex-based linkify in zulip does not allow '@' as the
prefix. In any case we know here that it is definitely a changelist
number, which is better.
Adding P4_WEB="https://p4web.example.com" to the config will enable
this behaviour. If P4_WEB is absent from the config, or has the
value None, no link is inserted.
This is designed to be run as a "change-commit" trigger. See [Perforce's
documentation][1] on how to set up a trigger for your platform.
[1]: http://www.perforce.com/perforce/r12.1/manuals/cmdref/triggers.html
Closes trac #1034.
(imported from commit d94fa4e50637ade2847a96eab8c5514de3811c24)
