.gitconfig: another top/new alias

Got a new answer to the corresponding SO question
(http://stackoverflow.com/a/18966319/131120) which seems much more advanced. It
uses a general reference for the upstream, which is labeled @{u} and it uses
commit ranges to state more elegantly what should be filtered in the log.

@{u}.. - should be the same as @{u}..HEAD or e.g. origin/master..master and it
         shows everything that is in master, but not in origin/master.

..@{u} - should be the same as HEAD..@{u} or e.g. master..origin/master and
         shows everything that is in the remote master but not in the local
         one.

Signed-off-by: Erik Bernoth <erik.bernoth@gmail.com>