Update format-diff script to add text to new files

[jaykorean]

Summary

Fixing internal validator failure

Every project specific source file must contain a doc block with an appropriate copyright header. Unrelated files must be listed as exceptions in the Copyright Headers Exceptions page in the repo dashboard.
A copyright header clearly indicates that the code is owned by Meta. Every open source file must start with a comment containing "Meta Platforms, Inc. and affiliates"
https://github.com/facebook/rocksdb/blob/main/buckifier/targets_cfg.py:
The first 16 lines of 'buckifier/targets_cfg.py' do not contain the patterns:
	(Meta Platforms, Inc. and affiliates)|(Facebook, Inc(\.|,)? and its affiliates)|([0-9]{4}-present(\.|,)? Facebook)|([0-9]{4}(\.|,)? Facebook)

While fixing the text to pass the linter, I took the opportunity to modify format-diff.sh script to add the copyright header automatically if missing in new files.

Test Plan

$> make format

new python file

build_tools/format-diff.sh
Checking format of uncommitted changes...
Checking for copyright headers in new files...
Added copyright header to build_tools/test.py
Copyright headers were added to new files.
Nothing needs to be reformatted!

new header file

build_tools/format-diff.sh
Checking format of uncommitted changes...
Checking for copyright headers in new files...
Added copyright header to db/db_impl/db_impl_jewoongh.h
Copyright headers were added to new files.
Nothing needs to be reformatted!

[meta-codesync]

@jaykorean has imported this pull request. If you are a Meta employee, you can view this in D87653124.

[meta-codesync]

@jaykorean merged this pull request in c4bbad4.