git-p4: restructure code in submit

git-p4: restructure code in submit

In preparation for adding new hooks to the submit method of git-p4,
restructure the applyCommit function in the P4Submit class.  Make the
following changes:

* Move all the code after the definition of submitted = False into the
  Try-Finally block. This ensures that any error that occurs will
  properly recover.  This is not necessary with the current code because
  none of it should throw an exception, however the next set of changes
  will introduce exceptional code.

  Existing flow control can remain as defined - the if-block for
  prepare-p4-only sets the local variable "submitted" to True and exits
  the function. New early exits, leave submitted set to False so the
  Finally block will undo changes to the P4 workspace.

* Make the small usability change of adding an empty string to the
  print statements displayed to the user when the prepare-p4-only option
  is selected.  On Windows, the command print() may display a set of
  parentheses "()" to the user when the print() function is called with
  no parameters. By supplying an empty string, the intended blank line
  will print as expected.

* Fix a small bug when the submittedTemplate is edited by the user
  and all content in the file is removed. The existing code will throw
  an exception if the separateLine is not found in the file. Change this
  code to test for the separator line using a find() test first and only
  split on the separator if it is found.

* Additionally, add the new behavior that if the changelist file has
  been completely emptied that the Submit action for this changelist
  will be aborted.

Signed-off-by: Ben Keene <seraphire@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>