Fix diff_drive accel limit (#242) (backport #252)

[mergify]

This is an automatic backport of pull request #252 done by Mergify.
Cherry-pick of 0469455 has failed:

On branch mergify/bp/foxy/pr-252
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	both modified:   diff_drive_controller/src/diff_drive_controller.cpp

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[joshnewans]

I don't know how this mergify business works, but presumably the issue here is that last_msg was renamed to last_command_msg somewhere between the foxy and galactic branches (something I had to change after making my initial fix on foxy and then cherry-picking to galactic).

[destogl]

@joshnewans thanks for the comment. Do you have an idea how to fix this fast? Can you propose a change directly on the files here?

@joshnewans I just guessed something. Can you check/review it, please?

[joshnewans]

Apart from that one change I flagged it looks good to me.