rev-parse: document --is-inside-git-dir

Signed-off-by: Matthias Lederhofer <matled@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-rev-parse.txt

@@ -89,6 +89,10 @@ OPTIONS
 --git-dir::
 	Show `$GIT_DIR` if defined else show the path to the .git directory.
 
+--is-inside-git-dir::
+	When the current working directory is below the repository
+	directory print "true", otherwise "false".
+
 --short, --short=number::
 	Instead of outputting the full SHA1 values of object names try to
 	abbreviate them to a shorter unique name. When no length is specified