54ce101 changed the way undo entries are created when adding decorations.
This creates all sorts of problems.This change fixes the problem by
reverting to the previous behavior.
Problem: When Normal highlight group defines ctermfg/bg, but other
highlight group lacks ctermfg/bg, nvim_get_hl_by_id(hl_id,
v:false) returns -1 for the missing ctermfg/bg instead of just
omitting it.
Solution: checking for -1 in hlattrs2dict()
fix#11680
...in order to retrieve highlights.
Added test/functional/api/highlight_spec.lua
HL_NORMAL is not really a good name, since it's more like an empty attribute than the normal's one.
If one pays attention, syn_cterm_attr2entry is never called with attr=0 because it's always special cased before.
I suggest in subsequent PRs we remove the ATTR_OFF and just insert an EMPTY ATTR/RESET_ATTR/UNINITIALIZED for id 0.
