Have a command that specifically invokes 'p4' (via system)

Similiar to our 'p4_read_pipe_lines' command, we can isolate
 specific changes to the invocation method in the one location
 with this change.
Signed-off-by: NAnand Kumria <wildfire@progsoc.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

contrib/fast-import/git-p4

@@ -70,6 +70,13 @@ def system(cmd):
     if os.system(cmd) != 0:
         die("command failed: %s" % cmd)
 
+def p4_system(cmd):
+    """Specifically invoke p4 as the system command. """
+    real_cmd = "%s %s" % ("p4", cmd)
+    if verbose:
+        print real_cmd
+    return system(real_cmd)
+
 def isP4Exec(kind):
     """Determine if a Perforce 'kind' should have execute permission