Make fread/fwrite-like functions in http.c more like fread/fwrite.

Make fread/fwrite-like functions in http.c more like fread/fwrite.

The fread/fwrite-like functions in http.c, namely fread_buffer,
fwrite_buffer, fwrite_null, fwrite_sha1_file all return the
multiplication of the size and number of items they are being given.

Practically speaking, it doesn't matter, because in all contexts where
those functions are used, size is 1.

But those functions being similar to fread and fwrite (the curl API is
designed around being able to use fread and fwrite directly), it might
be preferable to make them behave like fread and fwrite, which, from
the fread/fwrite manual page, is:
   On  success, fread() and fwrite() return the number of items read
   or written.  This number equals the number of bytes transferred
   only when size is 1.  If an error occurs, or the end of the file
   is reached, the return value is a short item count (or zero).

Signed-off-by: Mike Hommey <mh@glandium.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>