"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: NDavid Turner <dturner@twitter.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

On a case-insensitive filesystem, when merging, a file would be
wrongly deleted from the working tree if an incoming commit had
renamed it changing only its case.  When merging a rename, the file
with the old name would be deleted -- but since the filesystem
considers the old name to be the same as the new name, the new
file would in fact be deleted.

We avoid this by not deleting files that have a case-clone in the
index at stage 0.
Signed-off-by: NDavid Turner <dturner@twitter.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>