range-diff: publish default creation factor

The range-diff back-end allows its heuristic to be tweaked via the
"creation factor". git-range-diff, the only client of the back-end,
defaults the factor to 60% (hard-coded in builtin/range-diff.c), but
allows the user to override it with the --creation-factor option.

Publish the default range factor to allow new callers of the range-diff
back-end to default to the same value without duplicating the hard-coded
constant, and to avoid worrying about various callers becoming
out-of-sync if the default ever needs to change.
Signed-off-by: NEric Sunshine <sunshine@sunshineco.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/range-diff.c

@@ -18,7 +18,7 @@ static struct strbuf *output_prefix_cb(struct diff_options *opt, void *data)
 
 int cmd_range_diff(int argc, const char **argv, const char *prefix)
 {
-	int creation_factor = 60;
+	int creation_factor = RANGE_DIFF_CREATION_FACTOR_DEFAULT;
 	struct diff_options diffopt = { NULL };
 	int simple_color = -1;
 	struct option options[] = {

range-diff.h

@@ -3,6 +3,8 @@
 
 #include "diff.h"
 
+#define RANGE_DIFF_CREATION_FACTOR_DEFAULT 60
+
 int show_range_diff(const char *range1, const char *range2,
 		    int creation_factor, struct diff_options *diffopt);