build: don't use -s flag for `which` `which -s` is a BSDism that doesn't exist on other versions of `which`. That means that even on Mac OS X, `make osx` breaks if you have another utils package installed (e.g. debianutils installed thru fink). Redirect output to /dev/null instead.

contrib: drop obsolete sample.hgrc This was full of bad suggestions and is obsoleted by hg config --edit.