Adjust path parsed result for pplatex

[JingMatrix]

Latex log assumes project root to be the start directory

[JingMatrix]

I am still using pplatex, it seems their project needs some helps. I'll try to fix some problem there.
But let's fix the problem in vimtex first.

[lervag]

I don't see any reason not to accept, but I'll allow you to adjust as commented.

[JingMatrix]

I force updated my commit now, the first one is a rebase for your commits, and the second one is to adopt your advices.

Thanks for your advice on popd and pushd, it is intuitive just as the shell language.

[lervag]

Great, thanks! I'll merge now. :)