Further reduce the number of ways that functions run p4.
(This is more work in progress towards issue #7.)
In detail:
* Get rid of p4-exec-p4 and p4-read-depot-output.
* Get rid of unused second argument to p4-noinput-buffer-action.
* New function p4-run is now the only remaining function (other than p4-help-text) that uses call-process to run p4.

When you try to submit files with empty diffs, p4-submit now pops up a buffer listing these files. This fixes #11.