rhg: add rhg crate
The goal of rhg is to speedup some of hg's commands when possible by bypassing
python entirely for the time being.
It is by no means a replacement for hg as it will not support extentions or
configuration and implement only a subset of hg's commands and options.
Only use rhg if you understand what the tradeoffs are.
Differential Revision: https://phab.mercurial-scm.org/D8610
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.