git-p4: Change p4 command invocation

Change p4 command invocation to avoid going through the shell. This
allows names with spaces and wildcards to work.

Signed-off-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Vitor Antunes <vitor.hda@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/fast-import/git-p4

@@ -1984,7 +1984,7 @@ class P4Sync(Command, P4UserMap):
     def importChanges(self, changes):
         cnt = 1
         for change in changes:
-            description = p4Cmd("describe %s" % change)
+            description = p4Cmd(["describe", str(change)])
             self.updateOptionDict(description)
 
             if not self.silent: