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: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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);