Let 'interactive_shell' take a formula and set ENV vars.

Pull duplicate code out of install --debug and install -i up into
'interactive_shell'.

Library/Homebrew/formula.rb

@@ -203,8 +203,7 @@ class Formula
         puts "If nothing is installed or the shell exits with a non-zero error code,"
         puts "Homebrew will abort. The installation prefix is:"
         puts prefix
-        ENV['HOMEBREW_DEBUG_INSTALL'] = name
-        interactive_shell
+        interactive_shell self
       end
     end
   end

Library/Homebrew/install.rb

@@ -85,9 +85,7 @@ def install f
           puts "to copy the diff to the clipboard."
         end
 
-        ENV['HOMEBREW_DEBUG_PREFIX'] = f.prefix
-        ENV['HOMEBREW_DEBUG_INSTALL'] = f.name
-        interactive_shell
+        interactive_shell f
         nil
       elsif ARGV.include? '--help'
         system './configure --help'

Library/Homebrew/utils.rb

@@ -82,7 +82,12 @@ def pretty_duration s
   return "%.1f minutes" % (s/60)
 end
 
-def interactive_shell
+def interactive_shell f=nil
+  unless f.nil?
+    ENV['HOMEBREW_DEBUG_PREFIX'] = f.prefix
+    ENV['HOMEBREW_DEBUG_INSTALL'] = f.name
+  end
+
   fork {exec ENV['SHELL'] }
   Process.wait
   unless $?.success?