When indenting code, limit how far back we search.

Fixes https://github.com/JuliaLang/julia/issues/9254 -- performance is
just too bad on long files otherwise.
Adding unit tests for indentation.
Merge pull request #9308 from Wilfred/wh/julia_mode_lookback_limit

julia-mode.el: Set a maximum lookback for finding enclosing parens/blocks