"stg mail" does not consider the first patch in the series while generating the cover letter diffstats

[vlrpl]

stg mail doesn't include the first patch in the series to generate diffstats in a cover letter
when %(diffstat)s is present.

This is more visible generating a cover letter for a single patch.

Using e.g. the following cover letter template:

$ cat cover.txt
From: A U Thor <author@example.com>
Subject: Diffstat Cover Test

A diffstat cover test.

%(diffstat)s

The resulting mbox:
$ stg mail -m --cover=cover.txt $(stg top)

has an empty %(diffstat)s section.