Fix bash completion on mac.

For some bizarro reason, mac's bash doesn't like negative indexes,
even though its version of bash should support it.

src/etc/cargo.bashcomp.sh

@@ -125,7 +125,7 @@ __cargo_commands=$(cargo --list 2>/dev/null | tail -n +2)
 _locate_manifest(){
 	local manifest=`cargo locate-project 2>/dev/null`
 	# regexp-replace manifest '\{"root":"|"\}' ''
-	echo ${manifest:9:-2}
+	echo ${manifest:9:${#manifest}-11}
 }
 
 # Extracts the values of "name" from the array given in $1 and shows them as