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.h

@@ -0,0 +1,8 @@
+#ifndef INTERDIFF_H
+#define INTERDIFF_H
+
+struct rev_info;
+
+void show_interdiff(struct rev_info *);
+
+#endif