Detect uncompressed tars

From a patch by @RuiPereira

Closes Homebrew/homebrew#12011.

Library/Homebrew/download_strategy.rb

@@ -74,7 +74,7 @@ class CurlDownloadStrategy < AbstractDownloadStrategy
     when :zip
       quiet_safe_system '/usr/bin/unzip', {:quiet_flag => '-qq'}, @tarball_path
       chdir
-    when :gzip, :bzip2, :compress
+    when :gzip, :bzip2, :compress, :tar
       # Assume these are also tarred
       # TODO check if it's really a tar archive
       safe_system '/usr/bin/tar', 'xf', @tarball_path

Library/Homebrew/extend/pathname.rb

@@ -247,9 +247,10 @@ class Pathname
     # OS X installer package
     return :pkg if self.extname == '.pkg'
 
-    # get the first six bytes
+    # Get enough of the file to detect common file types
+    # POSIX tar magic has a 257 byte offset
     magic_bytes = nil
-    File.open(self) { |f| magic_bytes = f.read(6) }
+    File.open(self) { |f| magic_bytes = f.read(262) }
 
     # magic numbers stolen from /usr/share/file/magic/
     case magic_bytes
@@ -257,6 +258,7 @@ class Pathname
     when /^\037\213/     then :gzip
     when /^BZh/          then :bzip2
     when /^\037\235/     then :compress
+    when /^.{257}ustar/  then :tar
     when /^\xFD7zXZ\x00/ then :xz
     when /^Rar!/         then :rar
     else