Bug 1174037 - Support installing Mercurial from pip; r=glandium

Previously, we used a PPA on Ubuntu to install Mercurial. The PPA has
proved to be unreliable. Furthermore, we didn't have a mechanism for
installing a modern Mercurial on Debian and other derived distros.

In this commit, we remove the PPA from Ubuntu. We add the ability to
install Mercurial from pip to the Debian bootstrapper. However, since
some people may not want <not apt> installing package-like things,
we add a prompt explaining the situation and giving users a choice.
We recommend installing a modern Mercurial via pip. But we also given
the option to install a likely legacy version via apt. And, for Git
users, we give the option to not install Mercurial at all.

Since the new version of the Ubuntu bootstrapper is empty, it doesn't
need to exist, so it has been removed.

DONTBUILD (NPOTB)

python/mozboot/bin/bootstrap.py

@@ -44,7 +44,6 @@ REPOSITORY_PATHS = [
     'mozboot/gentoo.py',
     'mozboot/openbsd.py',
     'mozboot/osx.py',
-    'mozboot/ubuntu.py',
 ]
 
 TEMPDIR = None