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>