Use PRIuMAX instead of 'unsigned long long' in show-index

Use PRIuMAX instead of 'unsigned long long' in show-index

Elsewhere in Git we already use PRIuMAX and cast to uintmax_t when
we need to display a value that is 'very big' and we're not exactly
sure what the largest display size is for this platform.

This particular fix is needed so we can do the incredibly crazy
temporary hack of:

    diff --git a/cache.h b/cache.h
    index e0abcd6..6637fd8 100644
    --- a/cache.h
    +++ b/cache.h
    @@ -6,6 +6,7 @@

     #include SHA1_HEADER
     #include <zlib.h>
    +#define long long long

     #if ZLIB_VERNUM < 0x1200
     #define deflateBound(c,s)  ((s) + (((s) + 7) >> 3) + (((s) + 63) >> 6) + 11)

allowing us to more easily look for locations where we are passing
a pointer to an 8 byte value to a function that expects a 4 byte
value.  This can occur on some platforms where sizeof(long) == 8
and sizeof(size_t) == 4.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>