format-patch: add --range-diff 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 a range-diff, however, doing so involves manually
copy/pasting the diff into the cover letter.

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

    git format-patch --cover-letter --range-diff=v1 -3 v2

(At this stage, the previous attempt and the patch series being
formatted must share a common base, however, a subsequent enhancement
will make it possible to specify an explicit revision range for the
previous attempt.)

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

t/t3206-range-diff.sh

@@ -142,4 +142,16 @@ test_expect_success 'changed message' '
 	test_cmp expected actual
 '
 
+for prev in topic
+do
+	test_expect_success "format-patch --range-diff=$prev" '
+		git format-patch --stdout --cover-letter --range-diff=$prev \
+			master..unmodified >actual &&
+		grep "= 1: .* s/5/A" actual &&
+		grep "= 2: .* s/4/A" actual &&
+		grep "= 3: .* s/11/B" actual &&
+		grep "= 4: .* s/12/B" actual
+	'
+done
+
 test_done