fix(#1740): set open state only on folders

[alfa07]

Fixes issue 1740

If we set node.open indiscriminately (e.g. for files) then we fail later on this line when we try to draw the tree.

First we will recurse here on a file:
https://github.com/nvim-tree/nvim-tree.lua/blob/master/lua/nvim-tree/renderer/builder.lua#L263-L267

And then tree.node will be nil here:
https://github.com/nvim-tree/nvim-tree.lua/blob/master/lua/nvim-tree/renderer/builder.lua#L285

[alfa07]

@alex-courtis @kyazdani42 do you think this is the right way to fix the issue? Another option is to fix the check of node.open in here to is_folder and node.open.

[alex-courtis]

@alex-courtis @kyazdani42 do you think this is the right way to fix the issue? Another option is to fix the check of node.open in here to is_folder and node.open.

This fixes the root cause, which is the incorrect setting of node.open.

Let's not change the builder itself as there may be other assumptions around open and nodes.

[alex-courtis]

Fix tested OK however it did reveal an issue with the matcher: no-keep folders that begin with the path of a keep folder are left open:

:; mkdir 1740
:; cd 1740
:; mkdir dir dir1 dir2 dir3
:; echo 0 > dir/file0
:; echo 1 > dir1/file1
:; echo 2 > dir2/file2

If you could also resolve that in this PR I would be most grateful @alfa07

Edit: in a follow up PR

[alex-courtis]

Tested OK