Summary:
Usually when using cscope, the query results contain a lot of function calls in test, making it hard to browse. So this PR aims to provide an option to exclude test source files.

Add a new PHONY target, tags0, to exclude test source files while using cscope.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4190

Differential Revision: D9015901

Pulled By: riversand963

fbshipit-source-id: ea9a45756ccff5b26344d37e9ff1c02c5d9736d6