DirCacheEntry: Speed up creation by avoiding string cast

DirCacheEntry: Speed up creation by avoiding string cast

The checkPath function is available as a byte[] form, in fact the
String form just converts to byte[] to run the algorithm.

Having DirCacheEntry take a byte[] -> String -> byte[] to check if
each path is valid is a huge waste of CPU time. On some systems it
can double the time required to read 38,999 files from trees to the
DirCache. This slows down any operation using a DirCache.

Expose the byte[] form and use it for DirCacheEntry creation.

Change-Id: I6db7bc793ece99ff3c356338d793c07c061aeac7