docs: update long-help.txt with --quiet-empty flag

doc/long-help.txt

@@ -57,6 +57,10 @@ Options:
       --diff-context <N>
           Include N lines of context around added/removed/modified lines when using '--diff'.
 
+  -E, --quiet-empty
+          When this flag is set, bat will produce no output at all when the input is empty. This is
+          useful when piping commands that may produce empty output, like 'git diff'.
+
       --tabs <T>
           Set the tab width to T spaces. Use a width of 0 to pass tabs through directly