add_history_entry validates the entire tree

[braingram]

asdf/asdf/_asdf.py

Lines 1321 to 1325 in c0eae06

	try:
	self.validate()
	except Exception:
	self.tree["history"]["entries"].pop()
	raise

This appears to be done to validate the history entry. Perhaps there is way to just validate the new entry (or perhaps just the history portion of the file).