Use proper sed separator for paths

I941ef5ea90970a0901236afe81c551aaf24ac1d8 added a sed command that
should match and delete path values but used '/' as sed separator. This
leads to error in unstack.sh runs when the path also contains '/':

+./unstack.sh:main:188 sudo sed -i '/directory=/opt/stack/ d' /etc/gitconfig
sed: -e expression #1, char 13: unknown command: `o'

So this patch replace '/' separator with '+'.

Change-Id: I06811c0d9ee7ecddf84ef1c6dd6cff5129dbf4b1

unstack.sh

@@ -185,4 +185,4 @@ rm -Rf $DEST/async
 # Clean any safe.directory items we wrote into the global
 # gitconfig. We can identify the relevant ones by checking that they
 # point to somewhere in our $DEST directory.
-sudo sed -i "/directory=${DEST}/ d" /etc/gitconfig
+sudo sed -i "\+directory = ${DEST}+ d" /etc/gitconfig