Fix `brew edit` with environment filtering.

For many people `brew edit` makes use of the `EDITOR` variable to pick a
sensible editor. With environment filtering enabled unless this editor
is found in the default system PATH it'll fall back to e.g. `vim`.

Instead, ensure that we export the original, pre-filtering `PATH` as
`HOMEBREW_PATH` and use that internally to locate the editor. In future
this same approach will likely be used for requirements to be able to
find tools, too, and for other variables which we want to expose to
Homebrew itself but not other build tools.

Note that `HOMEBREW_PATH` is the same as `PATH` when build filtering
hasn't been enabled.

Library/Homebrew/test/utils_spec.rb

@@ -189,7 +189,13 @@ describe "globally-scoped helper methods" do
 
   specify "#which_editor" do
     ENV["HOMEBREW_EDITOR"] = "vemate"
-    expect(which_editor).to eq("vemate")
+    ENV["HOMEBREW_PATH"] = dir
+
+    editor = dir/"vemate"
+    FileUtils.touch editor
+    FileUtils.chmod 0755, editor
+
+    expect(which_editor).to eql editor
   end
 
   specify "#gzip" do

Library/Homebrew/utils.rb

@@ -320,8 +320,8 @@ def which_all(cmd, path = ENV["PATH"])
 end
 
 def which_editor
-  editor = ENV.values_at("HOMEBREW_EDITOR", "VISUAL", "EDITOR").compact.first
-  return editor unless editor.nil?
+  editor = ENV.values_at("HOMEBREW_EDITOR", "VISUAL").compact.first
+  return which(editor, ENV["HOMEBREW_PATH"]) unless editor.nil?
 
   # Find Textmate
   editor = "mate" if which "mate"
@@ -334,7 +334,7 @@ def which_editor
 
   opoo <<-EOS.undent
     Using #{editor} because no editor was set in the environment.
-    This may change in the future, so we recommend setting EDITOR, VISUAL,
+    This may change in the future, so we recommend setting EDITOR,
     or HOMEBREW_EDITOR to your preferred text editor.
   EOS
 

bin/brew

@@ -44,6 +44,14 @@ fi
 
 HOMEBREW_LIBRARY="$HOMEBREW_REPOSITORY/Library"
 
+for VAR in EDITOR PATH
+do
+  VAR_NEW="HOMEBREW_${VAR}"
+  # TODO: find a better solution than this.
+  env | grep -q "$VAR_NEW" && continue
+  export "$VAR_NEW"="${!VAR}"
+done
+
 if [[ -n "$HOMEBREW_ENV_FILTERING" ]]
 then
   PATH="/usr/bin:/bin:/usr/sbin:/sbin"