git-p4: document and test submit options

git-p4: document and test submit options

Clarify there is a -M option, but no -C.  These are both
configurable through variables.

Explain that the allowSubmit variable takes a comma-separated
list of branch names.

Catch earlier an invalid branch name given as an argument to
"git p4 clone".

Test option --origin, variable allowSubmit, and explicit master
branch name.

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