pushvars extension in fbext adds a --pushvars flag to push command using which
one send strings to server which becomes environment variables there prepended
with HG_USERVAR_. These variables can then be used to run hooks on the server.
The extension is moved directly to core and unbundling of the strings and
converting them to environment variables at server is disabled by default for
security reasons. One can turn that on by following config:
pushvars.server = true
This patch also adds the test for the extension.