--HEAD checks

* Force --HEAD for head-only installs
* Don't allow --HEAD for stable-only formulae

Library/Homebrew/cmd/install.rb

@@ -88,6 +88,16 @@ module Homebrew extend self
         # command line may have been installed to satisfy a dependency.
         next if f.installed? unless ARGV.force?
 
+        # Building head-only without --HEAD is an error
+        if not ARGV.build_head? and f.stable.nil?
+          raise "This is a head-only formula; install with `brew install --HEAD #{f.name}`"
+        end
+
+        # Building stable-only with --HEAD is an error
+        if ARGV.build_head? and f.unstable.nil?
+           raise "No head is defined for #{f.name}"
+        end
+
         begin
           fi = FormulaInstaller.new(f)
           fi.install