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