refactor: move apps listing from common plugin to apps plugin

This helps centralize app-related listing fetching.

plugins/git/install

@@ -18,7 +18,6 @@ trigger-git-install() {
 
 migrate_git_vars_0_12_0() {
   declare desc="migrates git config variables from 0.11.x"
-  local APPS="$(dokku_apps)"
   local DOKKU_DEPLOY_BRANCH app
 
   DOKKU_DEPLOY_BRANCH=$(config_get --global DOKKU_DEPLOY_BRANCH || true)
@@ -27,7 +26,7 @@ migrate_git_vars_0_12_0() {
     DOKKU_QUIET_OUTPUT=1 config_unset --global DOKKU_DEPLOY_BRANCH || true
   fi
 
-  for app in $APPS; do
+  for app in $(dokku_apps); do
     DOKKU_DEPLOY_BRANCH=$(config_get "$app" DOKKU_DEPLOY_BRANCH || true)
     if [[ -n "$DOKKU_DEPLOY_BRANCH" ]]; then
       fn-plugin-property-write "git" "$app" "deploy-branch" "$DOKKU_DEPLOY_BRANCH"