automation: install Rust in Linux environment
This will install Rust 1.31.1, 1.34.2, and whatever stable is at
the time the install runs. We install 1.31.1 as our minimum supported
Rust version (I think that's what we're currently targeting) and
1.34 because that's what Debian 10 is shipping.
Differential Revision: https://phab.mercurial-scm.org/D6715
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.