Branch of parent instead of submodule is displayed when editing `COMMIT_EDITMSG`

[spenserblack]

It looks like something similar has been brought up before (#2), but this is slightly different. When writing a commit message in a submodule, the parent's branch is displayed. This seems to be due to COMMIT_EDITMSG of a submodule existing in ../.git/modules/<submodule name>. It can be confusing to write a commit message when in a submodule and see the parent's branch.

According to :PlugSnapshot, I'm on commit 8118dc1

[itchyny]

Thanks for reporting, fixed.