notes: Initialise variable to appease gcc

notes: Initialise variable to appease gcc

gcc version 3.4.4 thinks that the 'cmp' variable could be used
while uninitialised and complains thus:

    notes.c: In function `write_each_non_note_until':
    notes.c:719: warning: 'cmp' might be used uninitialized in \
        this function

Note that gcc versions 4.1.2 and 4.4.0 do not issue this warning.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>