mktag: avoid %td in format string

mktag: avoid %td in format string

Since v1.7.0-rc0~34 (make "mktag" a built-in, 2010-01-22), git mktag
uses the C99-style %td format to print ptrdiff_t values.  It falls
back to %d when NO_C99_FORMAT is set, on the assumption that pre-C99
systems probably are using 32-bit pointers.

But many modern systems are 64-bit and

 * sometimes one wants to test the NO_C99_FORMAT fallbacks using a
   modern development platform;
 * some platforms (I'm looking at you, msvc) have not gotten with the
   program and are still C89-only.

These ptrdiff_t values are offsets from the beginning of a buffer, so
a size_t or uintmax_t would work about as well.  Use the latter so we
can take advantage of the PRIuMAX fallback in git-compat-util.h, even
on C99-challenged systems with 64-bit pointers.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>