range-diff: avoid compiler warning when char is unsigned

range-diff: avoid compiler warning when char is unsigned

Since 2b15969f61 (range-diff: let '--abbrev' option takes effect,
2023-02-20), GCC 11.3 on Ubuntu 22.04 on aarch64 warns (and errors
out if the make variable DEVELOPER is set):

range-diff.c: In function ‘output_pair_header’:
range-diff.c:388:20: error: comparison is always false due to limited range of data type [-Werror=type-limits]
  388 |         if (abbrev < 0)
      |                    ^
cc1: all warnings being treated as errors

That's because char is unsigned on that platform.  Use int instead, just
like in struct diff_options, to copy the value faithfully.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>