Fix occurrences of "cd `dirname $0`"

They are not safe for spaces in paths. There might be other places to look for trouble.

I've run the following command to produce this commit: (on OS X, copy-and-pasting the below exactly)

    find . -type f -name '*.sh' -print0  |  # Find all .sh files
        xargs -0 fgrep -H -- '`'         |  # See all places with backticks in them
        fgrep 'cd `dirname $0'           |  # I deemed these problematic (variable assignments are safe)
        cut -d ':' -f 1                  |  # Take the <file> from <file>:<line> produced by "grep -H"
        tr '\n' '\0'                     |  # Also here, spaces can be problematic - always do "xargs -0"!
        xargs -0 -- sed -i '' 's/cd `dirname $0`/cd "`dirname "$0"`"/g'

The significance of adding the two levels of "'s can be verified by running the following in your Terminal:

    $ node -e 'console.log(process.argv.splice(1))' -- `echo 1   2`
    [ '1', '2' ]

    $ node -e 'console.log(process.argv.splice(1))' -- "`echo 1   2`"
    [ '1 2' ]

    $ node -e 'console.log(process.argv.splice(1))' -- "`echo "1   2"`"
    [ '1   2' ]

I have not run or updated any tests, neither before nor after creating this commit.