add git-mv-recase test
mv.c: allow recasing a file on case preserving fs

On a case preserving fs, git mv will fail when you try to mv a file like
this...

$ git mv FileName.txt filename.txt

...because the filesystem thinks both files exist when in actuallity it's
the same file.

In the else if block where git checks to see if the destination exists,
also make sure the src and dest inodes are different.  If the inodes are
the same, allow the mv.