Core: Tiny efficiency fix to jQuery.extend / jQuery.fn.extend (#4246)
Core: Tiny efficiency fix to jQuery.extend / jQuery.fn.extend (#4246)

Read target[name] only when it's needed.

In addition to doing the property read-only when needed, this
avoids a slow path in V8 (see the issue for more details).

Fixes gh-4245
Closes gh-4246
jquery/jquery,roytoo/jquery,jquery/jquery,roytoo/jquery,jquery/jquery,roytoo/jquery