Fix issue #52 with files opened in tabs
Fix issue #52 with files opened in tabs

When opening several files in different tabs, e.g. with
`vim -p *.tex`, then the buffers are opened simultaneously.
This commit fixes an issue where the autogroup would be
reset on each new buffer, meaning that the first n-1 files
(or something similar) would have `&foldmethod == expr`.

kmarius/vimtex,Aster89/vimtex,kmarius/vimtex,lervag/vimtex,lervag/vimtex,Aster89/vimtex