Add brew doctor check for Cellar and TMP on separate volumes.

Add a brew doctor check for this issue:
http://github.com/mxcl/homebrew/issues/issue/1238

Library/Homebrew/brew_doctor.rb

+class Volumes
+  def initialize
+    @volumes = []
+    raw_mounts=`mount`
+    raw_mounts.split("\n").each do |line|
+      case line
+      when /^(.+) on (\S+) \(/
+        @volumes << [$1, $2]
+      end
+    end
+    # Sort volumes by longest path prefix first
+    @volumes.sort! {|a,b| b[1].length <=> a[1].length}
+  end
+
+  def which path
+    @volumes.each_index do |i|
+      vol = @volumes[i]
+      return i if is_prefix?(vol[1], path)
+    end
+
+    return -1
+  end
+end
+
+
+def is_prefix? prefix, longer_string
+  p = prefix.to_s
+  longer_string.to_s[0,p.length] == p
+end
+
+
 def check_for_stray_dylibs
   bad_dylibs = Dir['/usr/local/lib/*.dylib'].select { |f| File.file? f and not File.symlink? f }
   if bad_dylibs.length > 0
@@ -291,6 +322,35 @@ def check_for_symlinked_cellar
   end
 end
 
+def check_for_multiple_volumes
+  volumes = Volumes.new
+
+  # Find the volumes for the TMP folder & HOMEBREW_CELLAR
+  real_cellar = HOMEBREW_CELLAR.realpath
+  tmp=Pathname.new `/usr/bin/mktemp -d /tmp/homebrew-brew-doctor-XXXX`.strip
+  real_temp = tmp.realpath.parent
+
+  where_cellar = volumes.which real_cellar
+  where_temp = volumes.which real_temp
+
+  unless where_cellar == where_temp
+    puts <<-EOS.undent
+      Your Cellar and TMP folders are on different volumes.
+
+      Putting your Cellar and TMP folders on different volumes causes problems
+      for brews that install symlinks, such as Git.
+
+      Please post the details of your setup to this existing issue, if the comments
+      there don't already capture them:
+        http://github.com/mxcl/homebrew/issues/issue/1238
+
+      A work-around is available in this branch:
+        http://github.com/adamv/homebrew/tree/temp
+
+    EOS
+  end
+end
+
 def brew_doctor
   read, write = IO.pipe
 
@@ -312,6 +372,7 @@ def brew_doctor
     check_for_config_scripts
     check_for_dyld_vars
     check_for_symlinked_cellar
+    check_for_multiple_volumes
 
     exit! 0
   else