Make 'yo generate' overwrite yo_db file

[tyamagu2]

yo generate does not overwrite YOTemplate (yo_db) file if the file already exists. This PR makes it overwrite YOTemplate file. I'm not sure why it currently skips writing YOTemplate file, but this change certainly makes it easier to update yo_db.tpl of a project.

Note that it always overwrites YOTemplate content if the single file option is enabled, because Generator.files already has the file pointer when it tries to get the file pointer to write the content.

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[google-cla]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

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Individual signers

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Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
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[tyamagu2]

@googlebot I signed it!

[zchee]

@kazegusuri just ping

[kazegusuri]

hmm... I'm not sure why this skips... The mystery is derived from xo.