chg: allows default hg path to be overridden
Before this patch, chg will fall back to "hg" if neither CHGHG nor HG are set.
This may have trouble if the "hg" in PATH is not compatible with chg, which
can happen, for example, an old hg is installed in a virtualenv.
Since it's very hard to do a quick hg version check from chg, after discussion
in IRC with smf and marmoute, the quickest solution is to build a package with
a hardcoded absolute hg path in chg. This patch makes it possible by adding a
C macro HGPATH.
% creating repo
% add and commit
C test.py
% change
M test.py