addremove: add "ui." prefix to message color keys
I don't like fully-colorized status/warning messages, and I want to disable
them at all. If we'd supported a syntax like 'color.ui.*=none', I could
easily turn addremove.added/removed off as well as ui.error. This patch is
just for that.
Since addremove colors aren't released yet, which were added at ddc1da134772,
there are no compatibility concerns.
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