range-diff: avoid compiler warning when char is unsigned

Since 2b15969 (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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: rscharfe

range-diff.c

@@ -383,7 +383,7 @@ static void output_pair_header(struct diff_options *diffopt,
 	const char *color_new = diff_get_color_opt(diffopt, DIFF_FILE_NEW);
 	const char *color_commit = diff_get_color_opt(diffopt, DIFF_COMMIT);
 	const char *color;
-	char abbrev = diffopt->abbrev;
+	int abbrev = diffopt->abbrev;
 
 	if (abbrev < 0)
 		abbrev = DEFAULT_ABBREV;