rev-parse doc: pass "--" to rev-parse in the --prefix example

The "--" argument avoids "ambiguous argument: unknown revision or
path not in the working tree" errors when a pathname argument refers
to a non-existent file.

The "--" passed explicitly to set was removed because rev-parse
outputs the "--" argument that it is given.

Signed-off-by: Richard Hansen <hansenr@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-rev-parse.txt

@@ -91,7 +91,8 @@ repository.  For example:
 ----
 prefix=$(git rev-parse --show-prefix)
 cd "$(git rev-parse --show-toplevel)"
-eval "set -- $(git rev-parse --sq --prefix "$prefix" "$@")"
+# rev-parse provides the -- needed for 'set'
+eval "set $(git rev-parse --sq --prefix "$prefix" -- "$@")"
 ----
 
 --verify::