Add 'git-p4.allowSubmit' to git-p4

Add 'git-p4.allowSubmit' to git-p4

I'm working with a perforce repo using git-p4. There are some config
files which I need to change locally according to my environment. I'm
using a 'local' git branch to park these changes. And I want to avoid
accidentally checking them into p4 just by doing "git p4 submit"
mindlessly without realizing which branch I'm actually on.

This patch adds a new git config, 'git-p4.allowSubmit', which is a
whitelist of branch names. "git p4 submit" will only allow submissions
from local branches on the list. Useful for preventing inadvertently
submitting from a strictly local branch.

For backward compatibility, if this config is not set at all,
submissions from all branches are allowed.

Signed-off-by: Jing Xue <jingxue@digizenstudio.com>
Acked-By: Simon Hausmann <simon@lst.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>