Introduce remove_or_warn function

Introduce remove_or_warn function

This patch introduces the remove_or_warn function which is a
generalised version of the {unlink,rmdir}_or_warn functions.  It takes
an additional parameter indicating the mode of the file to be removed.

The patch also modifies certain functions to use remove_or_warn
where appropriate, and adds a test case for a bug fixed by the use
of remove_or_warn.

Signed-off-by: Peter Collingbourne <peter@pcc.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>