maint: choose editor in the commit-msg git hook the same way git does

maint: choose editor in the commit-msg git hook the same way git does

Git honours the GIT_EDITOR environment variable, the "core.editor" Git
configuration variable, and the EDITOR environment variable (in that
order, and defaulting to "vi" if none of them is set) to decide which
editor should be invoked for the user when he has to or want to edit
his commit message.

However, our commit-msg hook, when invoking an editor on behalf of the
user to allow him to fix a non-policy-complaint commit message, only
honoured the EDITOR environment variable.  To avoid potential annoying
inconsistencies, we should really use the same logic used by Git in the
selection of the editor.  Luckily, we don't have to duplicate this
logic (that would be brittle in the long term), as we can rely on the
"git var" command, designed exactly to be used in situations like this.

* scripts/git-hooks ($editor): Adjust definition.