Recommend interactive usage of fish_add_path

Since the `fish_add_path` command modifies a universal Fish variable
(which is automatically persisted to a file) it's unnecessary to run it
again every time a new shell is opened.

Library/Homebrew/test/utils/shell_spec.rb

@@ -98,7 +98,7 @@ describe Utils::Shell do
       ENV["SHELL"] = "/usr/local/bin/fish"
       ENV["fish_user_paths"] = "/some/path"
       expect(described_class.prepend_path_in_profile(path))
-        .to eq("echo 'fish_add_path #{path}' >> #{shell_profile}")
+        .to eq("fish_add_path #{path}")
     end
   end
 end

Library/Homebrew/utils/shell.rb

@@ -78,7 +78,7 @@ module Utils
       when :csh, :tcsh
         "echo 'setenv PATH #{csh_quote(path)}:$PATH' >> #{profile}"
       when :fish
-        "echo 'fish_add_path #{sh_quote(path)}' >> #{profile}"
+        "fish_add_path #{sh_quote(path)}"
       end
     end