msvc: Fix some "expr evaluates to function" compiler warnings

msvc: Fix some "expr evaluates to function" compiler warnings

In particular, the following warning is issued while compiling
notes.c:

    notes.c(927) : warning C4550: expression evaluates to a \
function which is missing an argument list

along with identical warnings on lines 928, 1016 and 1017.

In order to suppress the warning, we change the definition of
combine_notes_fn, so that the symbol type is an (explicit)
"pointer to function ...".  As a result, several other
declarations need some minor fix-up to take account of the
new typedef.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Acked-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>