feat(git): add aliases for merge sign and verify

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.

modules/git/init.zsh

@@ -106,6 +106,8 @@ alias glc='git shortlog --summary --numbered'
 alias gm='git merge'
 alias gmC='git merge --no-commit'
 alias gmF='git merge --no-ff'
+alias gmS='git merge -S'
+alias gmV='git merge --verify-signatures'
 alias gma='git merge --abort'
 alias gmt='git mergetool'