Merge branch 'mg/killed-merge' into maint

Killing "git merge --edit" before the editor returns control left
the repository in a state with MERGE_MSG but without MERGE_HEAD,
which incorrectly tells the subsequent "git commit" that there was
a squash merge in progress.  This has been fixed.

* mg/killed-merge:
  merge: save merge state earlier
  merge: split write_merge_state in two
  merge: clarify call chain
  Documentation/git-merge: explain --continue

diff --cc builtin/merge.c
Simple merge