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The mysql_install_db script is used to initialize the data directory.
However, if the user needs to apply some customized init commands (for
example to change user or password) they have to start the database
with `--skip-grant-tables` after the install script, and then restart
the database with normal mode.

To make it easier to include customization in the mysql_install_db
script, in this commit, a new parameter `extra-file` is added in
mysql_install_db script to support some extra customized init commands.

With this option we can support applying extra file containing sql for
mysql_install_db command line at runtime, which reduces the complexity
of customized initialization process.

This script is only used for install and is not included in the mtr
bootstrap file.

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer Amazon Web
Services, Inc.