Merge branch 'tb/document-status-u-tradeoff'

Suggest users to look into using--untracked=no option when "git
status" takes too long.

* tb/document-status-u-tradeoff:
  status: advise to consider use of -u when read_directory takes too long
  git status: document trade-offs in choosing parameters to the -u option

t/t7512-status-help.sh

@@ -14,6 +14,7 @@ test_description='git status advice'
 set_fake_editor
 
 test_expect_success 'prepare for conflicts' '
+	git config --global advice.statusuoption false &&
 	test_commit init main.txt init &&
 	git branch conflicts &&
 	test_commit on_master main.txt on_master &&