Huangminghuang/merge blocklog fix

[huangminghuang]

Change Description

This PR fixes two problems when using eosio-blocklog to merge block logs

• create the output directory if not exists.
• avoid checking blocks.log/blocks.index for blocks-dir when using merge-blocklogs option.

Change Type

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• Documentation

• Stability bug fix

• Other

• Other - special case

Testing Changes

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• New Tests

• Existing Tests

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Consensus Changes

• Consensus Changes

API Changes

• API Changes

Documentation Additions

• Documentation Additions