Add support for xz-compressed tarballs

Rationale: some software (e.g. GNU Coreutils, GnuTLS 3.x), have started
distributing _only_ xz-compressed tarballs. There is no system XZ
utility provided by OS X, but it is necessary so that we can continue to
provide formulae for this software.

If XZUtils isn't installed, we abort and prompt the user to `brew
install xz`.

The `xz` command itself doesn't do any untarring, so we write the
decompressed archive to stdout and pipe it to tar.

Library/Homebrew/download_strategy.rb

@@ -76,8 +76,8 @@ class CurlDownloadStrategy < AbstractDownloadStrategy
       # Use more than 4 characters to not clash with magicbytes
       magic_bytes = "____pkg"
     else
-      # get the first four bytes
+      # get the first six bytes
-      File.open(@tarball_path) { |f| magic_bytes = f.read(4) }
+      File.open(@tarball_path) { |f| magic_bytes = f.read(6) }
     end
 
     # magic numbers stolen from /usr/share/file/magic/
@@ -89,6 +89,10 @@ class CurlDownloadStrategy < AbstractDownloadStrategy
       # TODO check if it's really a tar archive
       safe_system '/usr/bin/tar', 'xf', @tarball_path
       chdir
+    when /^\xFD7zXZ\x00/ # xz compressed
+      raise "You must install XZutils: brew install xz" unless system "/usr/bin/which -s xz"
+      safe_system "xz -dc #{@tarball_path} | /usr/bin/tar xf -"
+      chdir
     when '____pkg'
       safe_system '/usr/sbin/pkgutil', '--expand', @tarball_path, File.basename(@url)
       chdir

Library/Homebrew/extend/pathname.rb

@@ -62,9 +62,9 @@ class Pathname
     return dst
   end
 
-  # extended to support the double extensions .tar.gz and .tar.bz2
+  # extended to support the double extensions .tar.gz, .tar.bz2, and .tar.xz
   def extname
-    /(\.tar\.(gz|bz2))$/.match to_s
+    /(\.tar\.(gz|bz2|xz))$/.match to_s
     return $1 if $1
     return File.extname(to_s)
   end

Library/Homebrew/test/test_bucket.rb

@@ -32,8 +32,10 @@ class BeerTasting < Test::Unit::TestCase
     assert_nothing_raised do
       MockFormula.new 'test-0.1.tar.gz'
       MockFormula.new 'test-0.1.tar.bz2'
+      MockFormula.new 'test-0.1.tar.xz'
       MockFormula.new 'test-0.1.tgz'
       MockFormula.new 'test-0.1.bgz'
+      MockFormula.new 'test-0.1.txz'
       MockFormula.new 'test-0.1.zip'
     end
   end