mv: allow renaming to fix case on case insensitive filesystems

"git mv hello.txt Hello.txt" on a case insensitive filesystem
always triggers "destination already exists" error, because these
two names refer to the same path from the filesystem's point of
view, and requires the user to give "--force" when correcting the
case of the path recorded in the index and in the next commit.

Detect this case and allow it without requiring "--force".

Signed-off-by: David Turner <dturner@twitter.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>