vim-patch:8.0.0914: highlight attributes are always combined (#10256)

Problem:    Highlight attributes are always combined.
Solution:   Add the 'nocombine' value to replace attributes instead of
            combining them. (scauligi, closes vim/vim#1963)
0cd2a94a40

Closes https://github.com/neovim/neovim/pull/10256.

src/nvim/highlight_defs.h

@@ -18,6 +18,7 @@ typedef enum {
   HL_UNDERCURL       = 0x10,
   HL_STANDOUT        = 0x20,
   HL_STRIKETHROUGH   = 0x40,
+  HL_NOCOMBINE       = 0x80,
 } HlAttrFlags;
 
 /// Stores a complete highlighting entry, including colors and attributes