checkout_merged(): squelch false warning from some gcc

checkout_merged(): squelch false warning from some gcc

gcc 4.6.2 (there may be others) does not realize that the variable "mode"
can never be used uninitialized in this function and issues a false warning
under -Wuninitialized option.

Squelch it with an unnecessary initialization; it is not like a single
assignment matters to the performance in this codepath that writes out
to the filesystem with checkout_entry() anyway.

Signed-off-by: Junio C Hamano <gitster@pobox.com>