[LIT] Decode string result in lit.util.capture

Summary: I think this is probably a bug, but I'm putting this up for review just to be sure. I think that `lit.util.capture` should decode the resulting string in the same way `lit.util.executeCommand` does.

Reviewers: ddunbar, EricWF

Reviewed By: EricWF

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6769

llvm-svn: 225681