git-annex-config --set/--unset: No longer change the local git config setting

git-annex-config --set/--unset: No longer change the local git config setting

e53070c1f quietly made it set the local git config too, but that was never
documented anywhere, and it had surprising results. If I set
annex.largefiles globally in a repo, I would expect to be able to change it
in another repo, and the original repo would get the change and use it,
rather than being stuck on the old value set there.

And, if I have a local annex.largefiles and set a different global default,
I'd be surprised to have my local setting overwritten.

annex.securehashesonly does need to be set locally, since it's a security
feature and the global is only a default until it gets set locally. So
special cased.