Enable Git 2.0's automatic commit signing

See previous comments on this topic for context, and see the relevant
sections from the Git 2.0 release notes [1].

 - Enable auto-signing with `git config --global commit.gpgsign true`.
   Note that this option means that rebased commits are also re-signed
   on the fly.

 - Re-introduce `git logsig` as an alias for `git log --show-signature`
   for more convenient display of signed commits

[1]: http://article.gmane.org/gmane.comp.version-control.git/250341