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Fix #247 servlet.CommitLogHook.onPostReceive takes minutes

When pushing tags, command.getOldId value is
“0000000000000000000000000000000000000000”.
So, can not get commit logs.
Merge pull request #286 from maliayas/patch-2

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Merge pull request #320 from kunigaku/fix_247

Fix #247 servlet.CommitLogHook.onPostReceive takes minutes
(refs #326)SSH clone URL includes username for logged-in users and it's hidden for non logged-in users.
Update README.md