selftests/signedoff-check.sh: ignore merge commits

When CI systems work on branches for testing, they usually create
merge commits.  And those won't be signed, specially not from the
author of the other commits.

Let's skip the merge commits to work around that.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

selftests/signedoff-check.sh

 #!/bin/sh -e
 
-AUTHOR="$(git log -1 --pretty='format:%aN <%aE>')"
-git log -1 --pretty=format:%B | grep "Signed-off-by: $AUTHOR"
+AUTHOR="$(git log --no-merges -1 --pretty='format:%aN <%aE>')"
+git log --no-merges -1 --pretty=format:%B | grep "Signed-off-by: $AUTHOR"
 if [ $? != 0 ]; then
     echo "The commit message does not contain author's signature (Signed-off-by: $AUTHOR)"
     return 1