Add 'git rev-parse --show-toplevel' option.

Shows the absolute path of the top-level working directory.

Signed-off-by: Steven Drake <sdrake@xnet.co.nz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/git-rev-parse.txt b/Documentation/git-rev-parse.txt
index 82045a2..dc829b3 100644
--- a/Documentation/git-rev-parse.txt
+++ b/Documentation/git-rev-parse.txt
@@ -112,6 +112,9 @@
 --remotes::
 	Show tag refs found in `$GIT_DIR/refs/remotes`.
 
+--show-toplevel::
+	Show the absolute path of the top-level directory.
+
 --show-prefix::
 	When the command is invoked from a subdirectory, show the
 	path of the current directory relative to the top-level