git-p4: clean up after p4 submit failure

git-p4: clean up after p4 submit failure

When "p4 submit" command fails in P4Submit.applyCommit, the
workspace is left with the changes.  We already have code to revert
the changes to the workspace when the user decides to cancel
submission by aborting the editor that edits the change description,
and we should treat the "p4 submit" failure the same way.

Clean the workspace if p4_write_pipe raised SystemExit, so that the
user don't have to do it themselves.

Signed-off-by: GIRARD Etienne <egirard@murex.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Jeff King <peff@peff.net>