When editing Meshes with vertex keys (RVKs), the undo stack isn't
updated correctly, causing a loss of the situation when entering
editmode after changing active key, and doing undo immediate after.
This due to an 'optimizer' in the undo code that tried to prevent undos
pushed on identical situations (e.g. pressing TAB multiple times).
Decided to remove that convention, the annoyance of having possible
multiple undos in a row isn't worth solving it by potential losses.
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