git-p4: avoid syncing duplicate changes

When a particular changeset affects multiple depot paths, it
will appear multiple times in the output of "p4 changes".
Filter out the duplicates to avoid the extra empty commits that
this otherwise would create.

Signed-off-by: Pete Wyckoff <pw@padd.com>
Acked-by: Simon Hausmann <simon@lst.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/fast-import/git-p4

@@ -442,13 +442,14 @@ def p4ChangesForPaths(depotPaths, changeRange):
     output = p4_read_pipe_lines("changes " + ' '.join (["%s...%s" % (p, changeRange)
                                                         for p in depotPaths]))
 
-    changes = []
+    changes = {}
     for line in output:
-        changeNum = line.split(" ")[1]
-        changes.append(int(changeNum))
+	changeNum = int(line.split(" ")[1])
+	changes[changeNum] = True
 
-    changes.sort()
-    return changes
+    changelist = changes.keys()
+    changelist.sort()
+    return changelist
 
 class Command:
     def __init__(self):