remove with-git-editor macro

Remove `with-git-editor'.  It's just like `with-editor' but defaults
to a different environment variable.  Using the latter and explicitly
specifying GIT_EDITOR is not really any worse.  The former also did
bind `magit-process-popup' to -1 but since that is specific to Magit,
which furthermore only used this macro once, this is overkill and
would just complicate the documentation.
implement the looping editor

Implement the "looping editor" as a fallback when the Emacsclient
cannot be used, i.e. for remote processes and for local processes
when we failed to find a suitable Emacsclient.
Merge branch 'n/looping-editor' into next