Make emacsclient work as artemis editor

[Chris Warburton]

git-artemis makes files called /tmp/tmpXXX.txt for issues, opens them in
EDITOR, then uses the result to create the actual issue. Fair
enough. The problem is, we want to use emacsclient as EDITOR, and that
will treat /tmp/tmpXXX.txt as a text file (and rightly so!).

What we need to do is open the file in emacsclient, then trigger some
Emacs Lisp to set the mode. Note that we can't do this with a single
emacsclient call, since opening files and evaluating code are treated as
mutually exclusive options.

We must open the file first, so that the buffer's available when we run
our ELisp. However, our ELisp evaluation will exit immediately, rather
than waiting for us to finish editing the file.

So, the solution seems to be:

 - Write a wrapper around git-artemis. Might as well call it
   "artemis". This will set the EDITOR to a custom script.
 - The custom script accepts a filename as $1, then calls emacsclient
   with that filename, using & to run it in the background.
 - We sleep for a second, to give Emacs chance to open the file, then we
   run emacsclient again, passing '--eval "(some-custom-lisp \"$1\")"'
   where some-custom-lisp will switch to the given filename's buffer and
   set the right mode.
 - We then need to `wait` for the first emacsclient process to finish
   (i.e. wait until we've finished editing). Otherwise we'll exit
   immediately, artemis will see the unmodified text and abort (since it
   checks whether the text is modified). Meanwhile we'll be in Emacs,
   blissfully unaware that artemis has already finished, and the issue
   we're entering will just end up cluttering /tmp without being
   processed.