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/test_helper.bash

@@ -71,6 +71,18 @@ assert_output() {
   assert_equal "$expected" "$output"
 }
 
+# ShellCheck doesn't know about $output from Bats
+# shellcheck disable=SC2154
+assert_output_exists() {
+  [[ -n "$output" ]] || flunk "expected output, found none"
+}
+
+# ShellCheck doesn't know about $output from Bats
+# shellcheck disable=SC2154
+assert_output_empty() {
+  [[ -z "$output" ]] || flunk "expected no output, found $output"
+}
+
 # ShellCheck doesn't know about $output from Bats
 # shellcheck disable=SC2154
 assert_output_contains() {