git-p4: use HEAD~$n to find parent commit for unshelve

Found-by: Liu Xuhui (Jackson) <Xuhui.Liu@amd.com>
Signed-off-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-p4.py

@@ -4237,7 +4237,7 @@ class P4Unshelve(Command):
         """
 
         for parent in (range(65535)):
-            log = extractLogMessageFromGitCommit("{0}^{1}".format(starting_point, parent))
+            log = extractLogMessageFromGitCommit("{0}~{1}".format(starting_point, parent))
             settings = extractSettingsGitLog(log)
             if 'change' in settings:
                 return settings