Use pwd -P to work on systemlinked locations.

Since I have my jruby.git directory in a symlinked directory, I get a failure:

 1) Failure:
test_backquote1(TestKernel) [test/test_kernel.rb:589]:
<"/ssd/ratnikov/jruby.git"> expected but was
<"/usr/local/XXX/ratnikov/ssd/jruby.git">.

since Dir.pwd resolves system links and pwd apparently doesn't. Using pwd -P solves this issue.