Trying to do this would previously have the second one silently overwrite the
first one's.
We don't currently have this in lazygit, but I ran into the situation once
during development, and it can lead to bugs that are hard to diagnose.
Instead of holding a list of functions, we could also have added a panic in case
the function was set already; this would have been good enough for the current
state, and enough to catch mistakes early in the future. However, I decided to
allow multiple controllers to attach these functions, because I can't see a
reason not to.
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