Run the COMMIT_EDITMSG editor from git-z

[ejpcmac]

Rationale

To solve #27 and run commit-msg from git z commit, we need a way to actually run the hook after the user has edited the message, but before git commit is called. This means calling the editor from git z commit instead of git commit.

Concept

Let’s let the user edit the commit message before calling git commit, and ask git commit not to edit the message.