format-patch: add --interdiff option to embed diff in cover letter

When submitting a revised version of a patch series, it can be helpful
(to reviewers) to include a summary of changes since the previous
attempt in the form of an interdiff, however, doing so involves manually
copy/pasting the diff into the cover letter.

Add an --interdiff option to automate this process. The argument to
--interdiff specifies the tip of the previous attempt against which to
generate the interdiff. For example:

    git format-patch --cover-letter --interdiff=v1 -3 v2

The previous attempt and the patch series being formatted must share a
common base.

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

interdiff.c

@@ -0,0 +1,17 @@
+#include "cache.h"
+#include "commit.h"
+#include "revision.h"
+#include "interdiff.h"
+
+void show_interdiff(struct rev_info *rev)
+{
+	struct diff_options opts;
+
+	memcpy(&opts, &rev->diffopt, sizeof(opts));
+	opts.output_format = DIFF_FORMAT_PATCH;
+	diff_setup_done(&opts);
+
+	diff_tree_oid(rev->idiff_oid1, rev->idiff_oid2, "", &opts);
+	diffcore_std(&opts);
+	diff_flush(&opts);
+}