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