scm/git: more old OS X versions tweaks.

- unset IFS to fix version parsing on older Bashes
- reuse existing HOMEBREW_MACOS_VERSION if it’s available
- set rather than unset popup_stub

shims/scm/git: fix on old OS X versions.

dependency_collector: need xz dep on <= macOS 10.8

On older versions of OS X (i.e. pre-Mavericks) `xcode-select` is not
required to be set when using the CLT. Fix this behaviour so `brew
update` doesn't tell everyone on that configuration to `brew install
git`.

Update FAQ.md

Mountain Lion: http://opensource.apple.com/source/libarchive/libarchive-25.1/config.h
  `/* #undef HAVE_LIBLZMA */`

Mavericks: http://opensource.apple.com/source/libarchive/libarchive-29/config.h
  `#define HAVE_LIBLZMA 1`

### Is there a glossary of terms around?
All your terminology needs can be [found here](docs/Formula-Cookbook.md#homebrew-terminology). results in 404 error

Fix wrong ordering on when clauses in audit.rb

Refactor `puts_columns` to allow `$stderr.puts_columns`.

Use BSD tar's libarchive LZMA support if available

Rename tap-readme command to tap-new.

Avoid using an unnecessary `xz` dependency when it's not needed.

Write the README but also a `.travis.yml` file (and in future perhaps a
`Jenkinsfile`).

formula_installer: Remove obsolete hard dependency on cctools.

It looks like a rebase/merge conflict remmenet.

README.md: fix URL for jawshooah

Haggins -> Hagins

All things must come to an end eventually.

brew doctor: warn about SSL_CERT_FILE (#1284)

Multiple issues have been filed about this environment variable, hence
give a warning. #932 might make this unnecessary when merged, but
hopefully this warning can be merged quickly.