format-patch: extend --range-diff to accept revision range

When submitting a revised a patch series, the --range-diff option embeds
a range-diff in the cover letter showing changes since the previous
version of the patch series. The argument to --range-diff is a simple
revision naming the tip of the previous series, which works fine if the
previous and current versions of the patch series share a common base.

However, it fails if the revision ranges of the old and new versions of
the series are disjoint. To address this shortcoming, extend
--range-diff to also accept an explicit revision range for the previous
series. For example:

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

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

t/t3206-range-diff.sh

@@ -142,7 +142,7 @@ test_expect_success 'changed message' '
 	test_cmp expected actual
 '
 
-for prev in topic
+for prev in topic master..topic
 do
 	test_expect_success "format-patch --range-diff=$prev" '
 		git format-patch --stdout --cover-letter --range-diff=$prev \