Fix build failure reported by Tobias Markmann in bug 17203.

svn 1.8.0 emits an additional line matching 'URL:' in its 'svn info' command
('Relative URL:').
Changed the grep to match only the intended line so that a valid SVNVersion.inc
is generated.

The problem doesnt occur with the svn version I'm using (1.7.5) but Tobias has
confirmed that the change fixes the problem.

See http://llvm.org/bugs/show_bug.cgi?id=17203

llvm-svn: 190685