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>
diff --git a/Documentation/git-format-patch.txt b/Documentation/git-format-patch.txt
index e7f404b..425145f 100644
--- a/Documentation/git-format-patch.txt
+++ b/Documentation/git-format-patch.txt
@@ -243,10 +243,12 @@
 	As a reviewer aid, insert a range-diff (see linkgit:git-range-diff[1])
 	into the cover letter showing the differences between the previous
 	version of the patch series and the series currently being formatted.
-	`previous` is a single revision naming the tip of the previous
-	series which shares a common base with the series being formatted (for
+	`previous` can be a single revision naming the tip of the previous
+	series if it shares a common base with the series being formatted (for
 	example `git format-patch --cover-letter --range-diff=feature/v1 -3
-	feature/v2`).
+	feature/v2`), or a revision range if the two versions of the series are
+	disjoint (for example `git format-patch --cover-letter
+	--range-diff=feature/v1~3..feature/v1 -3 feature/v2`).
 
 --notes[=<ref>]::
 	Append the notes (see linkgit:git-notes[1]) for the commit