crosh: fix prompt redraws w/color

Use the \[ and \] markers around the color sequences so readline knows
which bytes are invisible.

BUG=chromium:500076
TEST=scrolling through history no longer redraws the prompt wrong

Change-Id: I6ec5518847a3e7286af955a4f3c10438897a2a61
Previous-Reviewed-on: https://chromium-review.googlesource.com/294851
(cherry picked from commit 398717de9d022792131994dfc5b3567297c44220)
Reviewed-on: https://chromium-review.googlesource.com/295970
Reviewed-by: Paul Stewart <pstew@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/crosh/crosh b/crosh/crosh
index 28f7840..ccb3e9c 100755
--- a/crosh/crosh
+++ b/crosh/crosh
@@ -1247,7 +1247,7 @@
 
   # Create a colorized prompt to make it easier to see commands start/finish.
   local prompt
-  prompt="$(printf '%bcrosh>%b ' '\033[1;33m' '\033[0m')"
+  prompt="$(printf '%bcrosh>%b ' '\001\033[1;33m\002' '\001\033[0m\002')"
 
   while [ -z "${exit}" ]; do
     if shell_read "${prompt}" LINE_; then