determine the top directory. This allows the tool to find the correct top
directory when you have something like:

   /x/llvm
   /x/llvm2
   /x/llvm3

Previously the scripts would always find /x/llvm even if you were in the
llvm2 or llvm3 directory because the pattern didn't allow the digits at
the end of the path.

llvm-svn: 26751