We removed prefilling the skipHook prefix in b102646b207 with the intention of
making it clearer that using the prefix in normal commits and typing `w` to skip
hooks are now two independent features.
It turns out that some people liked it with prefilling the prefix and perceive
it as a regression, so put it back in.
But only if we don't have a preserved message; this is an important use case,
when you try to make a normal commit, the hook fails, and then you want to make
the same commit with skipping the hook, but with the same message that you
already typed.
ced70903ec