Latex completion in string, fix #8983

[dhoegh]

This enables Latex completion in strings as suggested in #8983.
This pull request fixes the mess in #8988.

[stevengj]

Sounds good to me.

[tkelman]

@dhoegh would it be possible to disable the latex completion in circumstances where path completion is also running? There's a bit of a confusing conflict with Windows paths here:

|__/                   |  i686-w64-mingw32
julia> cd("D:\\U
\UUparrow      \UpEquilibrium  \Updownarrow    \Uuparrow
\UpArrowBar    \Uparrow        \Upsilon

when I don't have any folders under D:\ that begin with U. It seems like it would be better to not show any completions in this particular case if it's easy to tell when you're doing both string and path completion.

[dhoegh]

I have noticed it and it is a bit annoying. I think the solution is to disable it when two adjacent backslashes is present. This can be done by changing this https://github.com/JuliaLang/julia/blob/master/base/REPLCompletions.jl#L168 to a regex similar to https://github.com/JuliaLang/julia/blob/master/base/REPLCompletions.jl#L190. Do you think this is the solution then I will make a pull later today?

[tkelman]

Sounds like it's worth a try, you understand this part of the code much better than I do. I'm not sure where/how many times things get escaped as they make their way through the completion code, but I can try out whatever you propose.

[dhoegh]

I think it is double escaped but I can fix it later today my solution would be insert somthing like:

1<slashpos && !string[slashpos-1]=='\\'

at https://github.com/JuliaLang/julia/blob/master/base/REPLCompletions.jl#L169 i will check it later.