Merge branch 'jv/merge-nothing-into-void'

"git merge FETCH_HEAD" dereferenced NULL pointer when merging
nothing into an unborn history (which is arguably unusual usage,
which perhaps was the reason why nobody noticed it).

* jv/merge-nothing-into-void:
  merge: fix NULL pointer dereference when merging nothing into void

diff --cc builtin/merge.c
Simple merge

diff --cc t/t7600-merge.sh
Simple merge