Set HOMEBREW_DEBUG_INSTALL when in a shell during install -d|-i

When an "install -d formula" fails, and the user gets dropped into an
interactive shell, set the HOMEBREW_DEBUG_INSTALL env var to the name
for the formula that failed. Also set variable if the user requests an
interactive install in the first place.

Note that this may be different than the formula being installed, since
it may have been a dep that failed.

Also remove todo in utils; users can now look for HOMEBREW_DEBUG_INSTALL
in their prompt command, and adjust their prompts accordingly.

Library/Homebrew/formula.rb

@@ -215,6 +215,7 @@ 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
       end
     end

Library/Homebrew/install.rb

@@ -85,6 +85,7 @@ def install f
           puts "to copy the diff to the clipboard."
         end
 
+        ENV['HOMEBREW_DEBUG_INSTALL'] = f.name
         interactive_shell
         nil
       elsif ARGV.include? '--help'

Library/Homebrew/utils.rb

@@ -47,13 +47,7 @@ def pretty_duration s
 end
 
 def interactive_shell
-  fork do
-    # TODO make the PS1 var change pls
-    #brown="\[\033[0;33m\]"
-    #reset="\[\033[0m\]"
-    #ENV['PS1']="Homebrew-#{HOMEBREW_VERSION} #{brown}\W#{reset}\$ "
-    exec ENV['SHELL']
-  end
+  fork {exec ENV['SHELL'] }
   Process.wait
   unless $?.success?
     puts "Aborting due to non-zero exit status"