config: add schema for git.push configuration entry

src/commands/config-schema.json

@@ -195,6 +195,11 @@
                             }
                         }
                     ]
+                },
+                "push": {
+                    "type": "string",
+                    "description": "The remote to which commits are pushed",
+                    "default": "origin"
                 }
             }
         },