git-p4: ensure submit clientPath exists before chdir

git-p4: ensure submit clientPath exists before chdir

Submitting patches back to p4 requires a p4 "client".  This
is a mapping from server depot paths into a local directory.
The directory need not exist or be populated with files; only
the mapping on the server is required.  When there is no
directory, make git-p4 automatically create it.

[ reword description --pw ]

Signed-off-by: Gary Gibbons <ggibbons@perforce.com>
Signed-off-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>