Use stringWithFileSystemRepresentation: instead of...

stringWithUTF8String:
Header doc clean up.
Assert that file.path != NULL
Assertions go at the top.
Return nil if we can't create a file system path.
Merge pull request #353 from libgit2/gtdifffile-improvements

-stringWithFileSystemRepresentation instead of -stringWithUTF8String in GTDiffFile