debian: support a "chg-first" installation mechanism (hg is actually chg) This mechanism builds chg such that it looks for `hg` to be available at /usr/lib/mercurial/hg instead of in the $PATH as `hg`, and makes the `hg` in /usr/bin be a symlink to `chg`. It's important to note that the hg binary must continue to be named `hg`. If we wanted to instead place it at /usr/bin/pyhg or something similar, we would need to modify Mercurial to allow that basename. Failure to do so would break Mercurial's shell aliases that use `hg`, `chg`, or `$HG`. I don't know if we should ever have a setup like this be the default setup, but I'm willing to get more information on our experience with it for making such a determination. Actually making it the default might be rather involved, as we don't maintain the official debian packaging rules. Differential Revision: https://phab.mercurial-scm.org/D10020