Refactor: minor cleanup of async shell functions

After the previous refactors, the logic for async shell commands can
be simplified somewhat. There is likely room for further improvements,
but that is beyond the scope of this PR.

app.go

@@ -526,9 +526,8 @@ func (app *app) runShell(s string, args []string, prefix string) {
 	}
 
 	// We are running the command asynchroniously
-	switch prefix {
-	case "%":
-		shellSetPG(cmd)
+	if prefix == "%" {
+		shellSetPG(cmd) // TODO: Delete or document the reason to do this twice for % mode
 		if app.ui.cmdPrefix == ">" {
 			return
 		}
@@ -543,12 +542,10 @@ func (app *app) runShell(s string, args []string, prefix string) {
 		}
 		out = stdout
 		cmd.Stderr = cmd.Stdout
-		fallthrough
-	case "&":
-		shellSetPG(cmd)
-		err = cmd.Start()
 	}
 
+	shellSetPG(cmd)
+	err = cmd.Start()
 	if err != nil {
 		app.ui.echoerrf("running shell: %s", err)
 	}