snap: Add git to snap package for GitInfo

Add git into the staged packages, so that the git binary is available
and hugo --enableGitInfo works for users of the snap package.

Fixes gohugoio/hugo#3896

snapcraft.yaml

@@ -22,6 +22,7 @@ parts:
       - git
       - make
     stage-packages:
+      - git
       - python-pygments
     prepare: |
       export GOPATH=$(dirname $SNAPCRAFT_PART_INSTALL)/go