status: advise to consider use of -u when read_directory takes too long

Introduce advice.statusUoption to suggest considering use of -u to
strike different trade-off when it took more than 2 seconds to
enumerate untracked/ignored files.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/config.txt b/Documentation/config.txt
index d1de857..a16eda5 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -163,6 +163,10 @@
 		state in the output of linkgit:git-status[1] and in
 		the template shown when writing commit messages in
 		linkgit:git-commit[1].
+	statusUoption::
+		Advise to consider using the `-u` option to linkgit:git-status[1]
+		when the command takes more than 2 seconds to enumerate untracked
+		files.
 	commitBeforeMerge::
 		Advice shown when linkgit:git-merge[1] refuses to
 		merge to avoid overwriting local changes.