shellcheck: chutney-git-bisect.sh issues

Shellcheck (https://github.com/koalaman/shellcheck) finds the following issues:

In chutney-git-bisect.sh line 23:
    cd "$2"
    ^-- SC2164: Use 'cd ... || exit' or 'cd ... || return' in case cd fails.


In chutney-git-bisect.sh line 57:
    i=$[$i+1]
      ^-- SC2007: Use $((..)) instead of deprecated $[..]


In chutney-git-bisect.sh line 60:
i=$[$i-1]
  ^-- SC2007: Use $((..)) instead of deprecated $[..]