Merge branch 'jt/commit-redundant-scissors-fix'

Merge branch 'jt/commit-redundant-scissors-fix'

"git commit -v --cleanup=scissors" used to add the scissors line
twice in the log message buffer, which has been corrected.

* jt/commit-redundant-scissors-fix:
  commit: unify logic to avoid multiple scissors lines when merging
  commit: avoid redundant scissor line with --cleanup=scissors -v