git-p4: add p4 submit hooks

The git command "commit" supports a number of hooks that support
changing the behavior of the commit command.  The git-p4.py program only
has one existing hook, "p4-pre-submit".  This command occurs early in
the process.  There are no hooks in the process flow for modifying
the P4 changelist text programmatically.

Adds 3 new hooks to git-p4.py to the submit option.

The new hooks are:
* p4-prepare-changelist - Execute this hook after the changelist file
  has been created. The hook will be executed even if the
  --prepare-p4-only option is set.  This hook ignores the --no-verify
  option in keeping with the existing behavior of git commit.

* p4-changelist - Execute this hook after the user has edited the
  changelist. Do not execute this hook if the user has selected the
  --prepare-p4-only option. This hook will honor the --no-verify,
  following the conventions of git commit.

* p4-post-changelist - Execute this hook after the P4 submission process
  has completed successfully. This hook takes no parameters and is
  executed regardless of the --no-verify option.  It's return value will
  not be checked.

The calls to the new hooks: p4-prepare-changelist, p4-changelist,
and p4-post-changelist should all be called inside the try-finally
block.

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

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