Improve maven up-to-date.

[mshima]

This PR splits maven up-to-date checksum into 2.

1. Generate a checksum to compare against reference.
2. Generate a complete checksum (generated webapp included) for reference.

Fixes problems where the generated webapp mutated but not the source.
Eg: ./mvn -Pprod, npm run clean-www, jhipster --with-entities.

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• All continuous integration tests are green
• Tests are added where necessary
• The JDL part is updated if necessary
• jhipster-online is updated if necessary
• Documentation is added/updated where necessary
• Coding Rules & Commit Guidelines as per our CONTRIBUTING.md document are followed

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