msvc: Fix some compiler warnings

msvc: Fix some compiler warnings

In particular, using the normal (or production) compiler
warning level (-W3), msvc complains as follows:

.../sha1.c(244) : warning C4018: '<' : signed/unsigned mismatch
.../sha1.c(270) : warning C4244: 'function' : conversion from \
   'unsigned __int64' to 'unsigned long', possible loss of data
.../sha1.c(271) : warning C4244: 'function' : conversion from \
   'unsigned __int64' to 'unsigned long', possible loss of data

Note that gcc issues a similar complaint about line 244 when
compiling with -Wextra.

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