fix suggested branch creation command when detaching head

Doing:

$ git checkout HEAD^

Generates the following message:

|warning: you are not on ANY branch anymore.
|If you meant to create a new branch from the commit, you need -b to
|associate a new branch with the wanted checkout.  Example:
|  git checkout -b <new_branch_name> HEAD^

Of course if the user does as told at this point the created branch
won't be located at the expected commit.  Reword this message a bit to
avoid such confusion.
Signed-off-by: NNicolas Pitre <nico@cam.org>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

git-checkout.sh

@@ -156,9 +156,9 @@ then
 	if test -n "$oldbranch"
 	then
 		detach_warn="warning: you are not on ANY branch anymore.
-If you meant to create a new branch from the commit, you need -b to
-associate a new branch with the wanted checkout.  Example:
-  git checkout -b <new_branch_name> $arg"
+If you meant to create a new branch from this checkout, you may still do
+so (now or later) by using -b with the checkout command again.  Example:
+  git checkout -b <new_branch_name>"
 	fi
 elif test -z "$oldbranch" && test -n "$branch"
 then