reproducible Debian: write a longer mail to notify maintainers of their new unrep packages

I had a case of such mail being marked as spam, mostly due to the
too-low whitespace ration and the whole body being only an URL.

This might improve deliverability to others as well, besides stop being
boarderline for my own spamassassin.

|Content preview:  2020-05-23 22:24 https://tests.reproducible-builds.org/debian/unstable/i386/acmetool


|   changed from reproducible -> FTBR
|
|Content analysis details:   (5.0 points, 5.0 required)
|
| pts rule name              description
|---- ---------------------- --------------------------------------------------
|-0.5 BAYES_05               BODY: Bayes spam probability is 1 to 5%
|                            [score: 0.0112]
| 0.2 HEADER_FROM_DIFFERENT_DOMAINS From and EnvelopeFrom 2nd level
|                            mail domains are different
| 1.1 LOCALPART_IN_SUBJECT   Local part of To: address appears in
|                            Subject
| 0.0 SPF_NONE               SPF: sender does not publish an SPF Record
| 0.0 SPF_HELO_NONE          SPF: HELO does not publish an SPF Record
| 0.0 TVD_SPACE_RATIO        No description available.
| 1.0 BODY_URI_ONLY          Message body is only a URI in one line of text or
|                            for an image
| 0.7 SUBJ_OBFU_PUNCT_FEW    Possible punctuation-obfuscated Subject:
|                            header
| 2.4 TVD_SPACE_RATIO_MINFP  Space ratio

Signed-off-by: Mattia Rizzolo <mattia@debian.org>