Fix use of "perl -i" on Windows

The perldiag(1) has following to say about this:

"Can't do inplace edit without backup"

(F) You're on a system such as MS-DOS that gets confused if
you try reading from a deleted (but still opened) file. You
have to say -i.bak, or some such.

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>