git-backport: Fix sed errors with merge commits
git-backport returns an error on some merge request commits.
sed: invalid option -- '1'
diffline indicates the beginning of the diff in a file. A merge request
may not have a diff so the resulting value of diffline is -1 which causes
the error.
Check for a minimum value for diffline (0).
Signed-off-by: Prarit Bhargava prarit@redhat.com