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: NJing Xue <jingxue@digizenstudio.com>
Acked-By: NSimon Hausmann <simon@lst.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

contrib/fast-import/git-p4

@@ -687,6 +687,10 @@ class P4Submit(Command):
         else:
             return False
 
+        allowSubmit = gitConfig("git-p4.allowSubmit")
+        if len(allowSubmit) > 0 and not self.master in allowSubmit.split(","):
+            die("%s is not in git-p4.allowSubmit" % self.master)
+
         [upstream, settings] = findUpstreamBranchPoint()
         self.depotPath = settings['depot-paths'][0]
         if len(self.origin) == 0: