Merge branch 'hi/merge-verify-sig-config'

"git merge" learned to pay attention to merge.verifySignatures
configuration variable and pretend as if '--verify-signatures'
option was given from the command line.

* hi/merge-verify-sig-config:
  t5573, t7612: clean up after unexpected success of 'pull' and 'merge'
  t: add tests for pull --verify-signatures
  merge: add config option for verifySignatures