Use single BeforeLast() call in wxConfigPathChanger ctor.

Use single BeforeLast() call in wxConfigPathChanger ctor.

Use a single BeforeLast() call with the "rest" argument and avoid calling
AfterLast() laster in wxConfigPathChanger ctor.

This is a small optimization which may count because wxConfigPathChanger is
used in a lot of wxFileConfig functions but, even more importantly, this works
around a bug in g++ 4 optimized build when the name was not filled by
AfterLast() call correctly as apparently the optimizer decided it was not
used. The real cause of this compiler bug was difficult to find as it couldn't
be reproduced in a simple test case but this change avoids it and fixes
wxFileConfig unit test in optimized build.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65863 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775