Simply use find_exe('hg') as the default value for $HG and require to manually
set it if you have special requirements.
While the default will not always be 100% correct (i.e. the identical hg
version) for many users it is and for the others the hg executable found in
the PATH should do most things correctly.
Developers or other users with multiple installs can set $HG or run something
like util.set_hgexecutable in their shell or python scripts.
Additionally util.hgexecutable() is now available so extensions can access
the value with a public interface, too.
