extensions: change magic "shipped with hg" string
I've caught multiple extensions in the wild lying about being
'internal', so it's time to move the goalposts on people. Goalpost
moving will continue until third party extensions stop trying to
defeat the system.
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
for software developers.
Basic install:
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing:
$ make local # build for inplace usage
$ ./hg --version # should show the latest version
See https://mercurial-scm.org/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.