feat: flesh out git plugin further

- add `report` and `set` subcommands to the git plugin
- move configuring the DOKKU_DEPLOY_BRANCH to the git plugin
- implement configurable GIT_REV support
- migrate DOKKU_DEPLOY_BRANCH settings
- add a golang-compatible, shell based method of setting plugin configuration variables

Deprecates the community dokku-git-rev plugin.

Closes #2621
Refs dokku-community/dokku-git-rev#9

plugins/git/subcommands/report

@@ -0,0 +1,5 @@
+#!/usr/bin/env bash
+source "$PLUGIN_AVAILABLE_PATH/git/internal-functions"
+set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
+
+cmd-git-report "$@"