[git-j] Add support for globally-defined aliases.

When defining an alias, you can now pass -g or --global to make the
alias global. This will then automatically apply to all repositories,
unless overridden by an alias within the repository.

This can also be used to list global-only aliases (by running
`git j aliases -g') or to remove global aliases.

In default usage, both aliases defined in the local repository and
global aliases will be used for operations.