no MERGE_HEAD after octopus merge failure

[Max Ivanov <ivanov.maxim@xxxxxxxxx>]

Hi All,

I am using git 2.12.0 and it leaves no MERGE_HEAD once octopus merge
failed with conflicts. Is it intentional? Files have conflicts markers
and once resolved `git commit` creates just a normal commit, which is
very inconvenient and confusing.

Thanks