tests: run mvdan/shfmt on test runs

While I do not agree with _every_ style change, this will force Dokku to have consistent formatting across all shell scripts, which is arguably a Good Thing™.

The command used to reprocess everything is:

```shell
shfmt -l -bn -ci -i 2 -w .
```

plugins/git/deploy-source

@@ -1,5 +1,6 @@
 #!/usr/bin/env bash
-set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
+set -eo pipefail
+[[ $DOKKU_TRACE ]] && set -x
 
 git_deploy_source() {
   declare desc="git deploy-source plugin trigger"
@@ -10,8 +11,8 @@ git_deploy_source() {
 
   # bail if another source is detected
   if [[ -n "$STDIN" ]]; then
-  	echo "$STDIN"
-  	return
+    echo "$STDIN"
+    return
   fi
 
   if [[ -d "$DOKKU_ROOT/$APP/refs" ]]; then