Britney Spears had a weird Instagram outburst on Saturday night, mocking a “famous” person and labeling this mysterious personality a “piece of s**t.” Spears published her expletive-laden tirade beneath a sarcastic image, annotating a computerized portrait of a 17th-century noblewoman wearing dark shades. The 40-year-old pop artist unleashed her latest diatribe between publishing a pair of bizarre films of …