git-p4: disable-rebase: allow setting this via configuration

This just lets you set the --disable-rebase option with the
git configuration options git-p4.disableRebase. If you're
using this option, you probably want to set it all the time
for a given repo.

Signed-off-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-p4.py

@@ -1368,7 +1368,7 @@ class P4Submit(Command, P4UserMap):
         self.shelve = False
         self.update_shelve = list()
         self.commit = ""
-        self.disable_rebase = False
+        self.disable_rebase = gitConfigBool("git-p4.disableRebase")
         self.prepare_p4_only = False
         self.conflict_behavior = None
         self.isWindows = (platform.system() == "Windows")