[test] Added an error message check.
[cxxtags] Changed an error message.
[cxxtags_db_manager][env] Changed a command name. cxxtags_merger is renamed to cxxtags_db_manager.
[test] Added a test for -p option.
[test] Added a test for rebuild command.
[cxxtags_query] Added --partial option for "rebuild" mode.
[test][env] Updated tests.
[doc] Updated README.md.