Skip subject line length check for merge commits

Some merge requests (e.g. those created for release branches) include
merge commits.  Prevent Danger from warning about excessive subject line
length for merge commits.  (While the proper way to detect a merge
commit would be to check the 'parents' attribute of a commit object,
Danger Python does not seem to populate that attribute, so a simple
string search is performed on the commit subject instead.)

(cherry picked from commit bc42690c)
89 jobs for !4578 with michal/improve-danger-checks-v9_11 in 53 seconds (queued for 10 seconds)
detached
Status Job ID Name Coverage
  Precheck
passed #1411954
linux amd64
autoreconf

00:00:16

passed #1411956
linux amd64
clang-format

00:00:10

canceled #1411957
linux amd64
coccinelle

00:00:49

passed #1411958
linux amd64
danger

00:00:20

passed #1411955
linux amd64
misc

00:00:20

 
  Build
canceled #1411973
linux amd64
clang:asan

00:00:19

canceled #1411980
linux amd64
clang:buster:amd64

00:00:17

canceled #1411982
amd64 libvirt
clang:freebsd11:amd64

00:00:15

canceled #1411983
amd64 libvirt
clang:freebsd12:amd64

00:00:14

canceled #1411984
amd64 libvirt
clang:openbsd:amd64

00:00:14

canceled #1411975
linux amd64
clang:tsan

00:00:26

canceled #1411977
linux amd64
clang:tsan:noatomics

00:00:24

canceled #1411959
linux amd64
gcc:alpine3.12:amd64

00:00:31

canceled #1411972
linux amd64
gcc:asan

00:00:24

canceled #1411965
linux amd64
gcc:buster:amd64

00:00:28

canceled #1411961
linux amd64
gcc:centos6:amd64

00:00:19

canceled #1411960
linux i386
gcc:centos6:i386

00:00:30

canceled #1411962
linux amd64
gcc:centos7:amd64

00:00:30

canceled #1411963
linux amd64
gcc:centos8:amd64

00:00:29

canceled #1411969
linux amd64
gcc:fedora33:amd64

00:00:27

canceled #1411971
linux amd64
gcc:focal:amd64

00:00:25

canceled #1411978
linux amd64
gcc:mutexatomics

00:00:22

canceled #1411979
linux amd64
gcc:noatomics

00:00:18

canceled #1411967
linux amd64
gcc:out-of-tree

00:00:27

canceled #1411981
linux amd64
gcc:pkcs11

00:00:16

canceled #1411966
linux amd64
gcc:sid:amd64

00:00:28

canceled #1411964
linux amd64
gcc:stretch:amd64

00:00:29

canceled #1411974
linux amd64
gcc:tsan

00:00:27

canceled #1411976
linux amd64
gcc:tsan:noatomics

00:00:25

canceled #1411968
linux amd64
gcc:tumbleweed:amd64

00:00:27

canceled #1411970
linux amd64
gcc:xenial:amd64

00:00:26

canceled #1411986
amd64 windows
msvc:windows:amd64

00:00:49

canceled #1411985
linux amd64
nolibtool:sid:amd64

00:00:16

 
  Unit
canceled #1412000
linux amd64
unit:clang:asan

canceled #1412007
linux amd64
unit:clang:buster:amd64

canceled #1412009
amd64 libvirt
unit:clang:freebsd11:amd64

canceled #1412010
amd64 libvirt
unit:clang:freebsd12:amd64

canceled #1412002
linux amd64
unit:clang:tsan

canceled #1412004
linux amd64
unit:clang:tsan:noatomics

canceled #1411987
linux amd64
unit:gcc:alpine3.12:amd64

canceled #1411999
linux amd64
unit:gcc:asan

canceled #1411993
linux amd64
unit:gcc:buster:amd64

canceled #1411989
linux amd64
unit:gcc:centos6:amd64

canceled #1411988
linux i386
unit:gcc:centos6:i386

canceled #1411990
linux amd64
unit:gcc:centos7:amd64

canceled #1411991
linux amd64
unit:gcc:centos8:amd64

canceled #1411996
linux amd64
unit:gcc:fedora33:amd64

canceled #1411998
linux amd64
unit:gcc:focal:amd64

canceled #1412005
linux amd64
unit:gcc:mutexatomics

canceled #1412006
linux amd64
unit:gcc:noatomics

canceled #1412008
linux amd64
unit:gcc:pkcs11

canceled #1411994
linux amd64
unit:gcc:sid:amd64

canceled #1411992
linux amd64
unit:gcc:stretch:amd64

canceled #1412001
linux amd64
unit:gcc:tsan

canceled #1412003
linux amd64
unit:gcc:tsan:noatomics

canceled #1411995
linux amd64
unit:gcc:tumbleweed:amd64

canceled #1411997
linux amd64
unit:gcc:xenial:amd64

canceled #1412011
linux amd64
unit:nolibtool:sid:amd64

 
  System
canceled #1412025
linux amd64
system:clang:asan

canceled #1412032
linux amd64
system:clang:buster:amd64

canceled #1412034
amd64 libvirt
system:clang:freebsd11:amd64

canceled #1412035
amd64 libvirt
system:clang:freebsd12:amd64

canceled #1412027
linux amd64 allowed to fail
system:clang:tsan

canceled #1412029
linux amd64 allowed to fail
system:clang:tsan:noatomics

canceled #1412012
linux amd64
system:gcc:alpine3.12:amd64

canceled #1412024
linux amd64
system:gcc:asan

canceled #1412018
linux amd64
system:gcc:buster:amd64

canceled #1412014
linux amd64
system:gcc:centos6:amd64

canceled #1412013
linux i386
system:gcc:centos6:i386

canceled #1412015
linux amd64
system:gcc:centos7:amd64

canceled #1412016
linux amd64
system:gcc:centos8:amd64

canceled #1412021
linux amd64
system:gcc:fedora33:amd64

canceled #1412023
linux amd64
system:gcc:focal:amd64

canceled #1412030
linux amd64
system:gcc:mutexatomics

canceled #1412031
linux amd64
system:gcc:noatomics

canceled #1412033
linux amd64
system:gcc:pkcs11

canceled #1412019
linux amd64
system:gcc:sid:amd64

canceled #1412017
linux amd64
system:gcc:stretch:amd64

canceled #1412026
linux amd64 allowed to fail
system:gcc:tsan

canceled #1412028
linux amd64 allowed to fail
system:gcc:tsan:noatomics

canceled #1412020
linux amd64
system:gcc:tumbleweed:amd64

canceled #1412022
linux amd64
system:gcc:xenial:amd64

canceled #1412036
linux amd64
system:nolibtool:sid:amd64

 
  Docs
canceled #1412037
linux amd64
docs

00:00:15

 
  Postcheck
canceled #1412041
linux amd64
cppcheck

00:00:07

canceled #1412038
linux amd64
flake8

00:00:08

canceled #1412042
linux amd64
gcov

canceled #1412039
linux amd64
pylint

00:00:10

canceled #1412040
linux amd64
scan-build

00:00:10