format-patch: run range-diff with larger creation-factor

We see too often that a range-diff added to format-patch output
shows too many "unmatched" patches.  This is because the default
value for creation-factor is set to a relatively low value.

It may be justified for other uses (like you have a yet-to-be-sent
new iteration of your series, and compare it against the 'seen'
branch that has an older iteration, probably with the '--left-only'
option, to pick out only your patches while ignoring the others) of
"range-diff" command, but when the command is run as part of the
format-patch, the user _knows_ and expects that the patches in the
old and the new iterations roughly correspond to each other, so we
can and should use a much higher default.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

range-diff.h

@@ -6,6 +6,12 @@
 
 #define RANGE_DIFF_CREATION_FACTOR_DEFAULT 60
 
+/*
+ * A much higher value than the default, when we KNOW we are comparing
+ * the same series (e.g., used when format-patch calls range-diff).
+ */
+#define CREATION_FACTOR_FOR_THE_SAME_SERIES 999
+
 struct range_diff_options {
 	int creation_factor;
 	unsigned dual_color:1;