range-diff: offer --left-only/--right-only options

When comparing commit ranges, one is frequently interested only in one
side, such as asking the question "Has this patch that I submitted to
the Git mailing list been applied?": one would only care about the part
of the output that corresponds to the commits in a local branch.

To make that possible, imitate the `git rev-list` options `--left-only`
and `--right-only`.

This addresses https://github.com/gitgitgadget/git/issues/206

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

range-diff.h

@@ -9,6 +9,7 @@
 struct range_diff_options {
 	int creation_factor;
 	unsigned dual_color:1;
+	unsigned left_only:1, right_only:1;
 	const struct diff_options *diffopt; /* may be NULL */
 	const struct strvec *other_arg; /* may be NULL */
 };