After opening a file, an immediate "u"/":undo" would remove the entire
contents of the buffer. Even worse, this would not set 'modified', so there
would be no indication that wasn't the actual initial state of the buffer.
Discarding undo history for a buffer requires a few steps, including making a
change. Luckily, a change is already being made (deleting the extra line left
after ":r !" the decrypted file), so discarding undo history can piggy-back on
top of that.
Signed-off-by: James McCoy <vega.james@gmail.com>
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