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

tests/unit/10_git.bats

@@ -0,0 +1,89 @@
+#!/usr/bin/env bats
+
+load test_helper
+
+setup() {
+  global_setup
+  create_app
+}
+
+teardown() {
+  destroy_app
+  global_teardown
+}
+
+@test "(git) deploy specific branch" {
+  run bash -c "dokku git:set --global deploy-branch global-branch"
+  echo "output: "$output
+  echo "status: "$status
+  assert_success
+
+  GIT_REMOTE_BRANCH=global-branch deploy_app
+  echo "output: "$output
+  echo "status: "$status
+  assert_success
+
+  run bash -c "dokku ps:rebuild $TEST_APP"
+  echo "output: "$output
+  echo "status: "$status
+  assert_success
+
+  run bash -c "dokku git:set $TEST_APP deploy-branch app-branch"
+  GIT_REMOTE_BRANCH=app-branch deploy_app
+  echo "output: "$output
+  echo "status: "$status
+  assert_success
+
+  run bash -c "dokku ps:rebuild $TEST_APP"
+  echo "output: "$output
+  echo "status: "$status
+  assert_success
+
+  run bash -c "dokku git:set --global deploy-branch"
+}
+
+@test "(git) ensure GIT_REV env var is set" {
+  deploy_app
+  echo "output: "$output
+  echo "status: "$status
+  assert_success
+
+  run bash -c "dokku config:get $TEST_APP GIT_REV"
+  echo "output: "$output
+  echo "status: "$status
+  assert_output_exists
+}
+
+@test "(git) disable GIT_REV" {
+  run bash -c "dokku git:set $TEST_APP rev-env-var"
+  echo "output: "$output
+  echo "status: "$status
+  assert_success
+
+  deploy_app
+  echo "output: "$output
+  echo "status: "$status
+  assert_success
+
+  run bash -c "dokku config:get $TEST_APP GIT_REV"
+  echo "output: "$output
+  echo "status: "$status
+  assert_output_empty
+}
+
+@test "(git) customize the GIT_REV environment variable" {
+  run bash -c "dokku git:set $TEST_APP rev-env-var GIT_REV_ALT"
+  echo "output: "$output
+  echo "status: "$status
+  assert_success
+
+  deploy_app
+  echo "output: "$output
+  echo "status: "$status
+  assert_success
+
+  run bash -c "dokku config:get $TEST_APP GIT_REV_ALT"
+  echo "output: "$output
+  echo "status: "$status
+  assert_output_exists
+}