dot: replace large labels by "(label too long)"

Based on a report by Victor Khomenko.

* spot/twaalgos/dot.cc: Here.
* tests/core/readsave.test: Add test case.
* NEWS: Mention it.
13 jobs for next in 129 minutes and 33 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #45635
alpine-gcc

00:25:13

passed #45636
arch-clang

00:42:43

passed #45637
arch-gcc-glibcxxdebug

00:50:56

passed #45634
debian-gcc-snapshot

01:16:20

passed #45632
debian-stable-gcc

01:00:29

passed #45633
debian-unstable-gcc-coverage

00:44:08

88.7%
passed #45640
debpkg-stable

01:59:39

passed #45641
debpkg-unstable

01:58:24

passed #45638
mingw-shared

00:13:14

passed #45639
mingw-static

00:08:37

passed #45642
rpm-pkg

00:47:17

 
  Publish
passed #45643
dput
publish-rpm

00:00:31

passed #45644
dput
publish-unstable

00:02:20