osx: add support for keeping mpkgs
This is a bit of a hack, but I don't really want to mount a dmg during
a test, and I don't see an option with hdiutil to take a dmg and spit
out a folder, so this is what we've got for now.
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
for software developers.
Basic install:
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing:
$ make local # build for inplace usage
$ ./hg --version # should show the latest version
See https://mercurial-scm.org/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.