Port "alias" symlinks to "alias" declarations in `.gitconfig`s

[ryan-williams]

After discussion with @danvk, I've turned against the pattern of expressing aliases of commands as symlinks in aliases/ directories that are also added to a user's path.

I think it would be better to declare them in .gitconfig files, one in each command-specific subdirectory.

The main theoretical "con" of this approach is that the "main" .gitconfig (which lives in config/ and is included from a user's ~/.gitconfig by .git-rc) will have to itself include many .gitconfig files, meaning every git command will do more file opens/reads.

However, I think that is a negligible cost for a much cleaner expression that uses git's well-supported "config alias" facility.