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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.
Update README.md

Ran into this issue today.  Added to documentation to let other users know about it.
Merge pull request #323 from r0n22/patch-1

Documentation Change for Large file Sizes
Fix #325
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