Adds the following aliases:
- gmS: GPG-sign the resulting merge commit.
- gmV: Verify that the tip commit of the side branch being merged is
signed with a valid key, i.e. a key that has a valid uid: in the
default trust model, this means the signing key has been signed by a
trusted key. If the tip commit of the side branch is not signed with a
valid key, the merge is aborted.
Closes#333
dc6ef566af