git-p4: avoid leak of file handle when cloning

Spotted by Eric Sunshine:

    https://public-inbox.org/git/CAPig+cRx3hG64nuDie69o_gdX39F=sR6D8LyA7J1rCErgu0aMA@mail.gmail.com/

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

git-p4.py

@@ -3563,7 +3563,8 @@ class P4Sync(Command, P4UserMap):
         changes = []
 
         if len(self.changesFile) > 0:
-            output = open(self.changesFile).readlines()
+            with open(self.changesFile) as f:
+                output = f.readlines()
             changeSet = set()
             for line in output:
                 changeSet.add(int(line))