git-p4: Avoid modules deprecated in Python 2.6.

The popen2, sha and sets modules are deprecated in Python 2.6 (sha in
Python 2.5).  Both popen2 and sha are not actually used in git-p4.
Replace usage of sets.Set with the builtin set object.

The built-in set object was added in Python 2.4 and is already used in
other parts of this script, so this dependency is nothing new.

Signed-off-by: Reilly Grant <reillyeon@qotw.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/fast-import/git-p4

@@ -8,12 +8,10 @@
 # License: MIT <http://www.opensource.org/licenses/mit-license.php>
 #
 
-import optparse, sys, os, marshal, popen2, subprocess, shelve
-import tempfile, getopt, sha, os.path, time, platform
+import optparse, sys, os, marshal, subprocess, shelve
+import tempfile, getopt, os.path, time, platform
 import re
 
-from sets import Set;
-
 verbose = False
 
 
@@ -864,8 +862,8 @@ class P4Sync(Command):
 
         self.usage += " //depot/path[@revRange]"
         self.silent = False
-        self.createdBranches = Set()
-        self.committedChanges = Set()
+        self.createdBranches = set()
+        self.committedChanges = set()
         self.branch = ""
         self.detectBranches = False
         self.detectLabels = False
@@ -1662,7 +1660,7 @@ class P4Sync(Command):
 
             if len(self.changesFile) > 0:
                 output = open(self.changesFile).readlines()
-                changeSet = Set()
+                changeSet = set()
                 for line in output:
                     changeSet.add(int(line))