Make Config.readIncludedConfig a noop by default

Make Config.readIncludedConfig a noop by default

The Config class must be safe to run against untrusted input files.
Reading arbitrary local system paths using include.path is risky for
servers, including Gerrit Code Review.  Return null by default to
incide the include should be ignored.

Only FileBasedConfig which originated from local disk should be trying
to read local system paths.  FileBasedConfig already overrides this
method with its own implementation.

Change-Id: I2ff31753868aa1bbac4a6843a4c23e50bd6f46f3