range-diff/format-patch: refactor check for commit range

Currently, when called with exactly two arguments, `git range-diff`
tests for a literal `..` in each of the two. Likewise, the argument
provided via `--range-diff` to `git format-patch` is checked in the same
manner.

However, `<commit>^!` is a perfectly valid commit range, equivalent to
`<commit>^..<commit>` according to the `SPECIFYING RANGES` section of
gitrevisions[7].

In preparation for allowing more sophisticated ways to specify commit
ranges, let's refactor the check into its own function.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

range-diff.c

@@ -564,3 +564,8 @@ int show_range_diff(const char *range1, const char *range2,
 
 	return res;
 }
+
+int is_range_diff_range(const char *arg)
+{
+	return !!strstr(arg, "..");
+}