Fix directory stack manipulation in bash

In bash, `~` expands to places in the directory stack, not `-`. These
are also accepted in zsh, so everything works.

shell/common/directories.sh

@@ -10,12 +10,12 @@ alias po='popd'
 # Access the directory stack.
 alias d='dirs -v | head -10'
 alias -- -='cd -'
-alias 1='cd -'
-alias 2='cd -2'
-alias 3='cd -3'
-alias 4='cd -4'
-alias 5='cd -5'
-alias 6='cd -6'
-alias 7='cd -7'
-alias 8='cd -8'
-alias 9='cd -9'
+alias 1='cd ~1'
+alias 2='cd ~2'
+alias 3='cd ~3'
+alias 4='cd ~4'
+alias 5='cd ~5'
+alias 6='cd ~6'
+alias 7='cd ~7'
+alias 8='cd ~8'
+alias 9='cd ~9'