Face +a attribute now merges with base +f and +g instead of overriding

The previous logic would just take the new face attributes if it had
"final attributes" on, but this is unlikely to be what we want, we
most likely want the "final fg" and "final bg" attributes of the base
face to be preserved.

Fixes #5415

Author: mawww

doc/pages/faces.asciidoc

@@ -61,7 +61,9 @@ attributes::
                 Same as final, but only applies to a face's background color.
         *a*:::
             final attributes::::
-                Same as final, but only applies to a face's attributes.
+                Same as final, but only applies to a face's attributes, note
+                that this will not clear to the f/g/a attributes of the base
+                face, so `+a` cannot be used to remove `+f` and/or `+g`
 
 base::
     The face onto which other faces apply. Its value can be any face name,

test/regression/5415-final-fg-attributes-lost-to-final-attr/cmd

@@ -0,0 +1 @@
+

test/regression/5415-final-fg-attributes-lost-to-final-attr/in

@@ -0,0 +1,2 @@
+a <b c> d
+

test/regression/5415-final-fg-attributes-lost-to-final-attr/rc

@@ -0,0 +1,9 @@
+face global Whitespace yellow+f
+addhl global/ show-whitespaces -spc .
+face global before_underline cyan
+face global add_underline ,,magenta+c
+face global remove_underline +a
+addhl global/a regex <([^>]*)> 1:before_underline
+addhl global/b regex [a-z] 0:add_underline
+addhl global/c regex <([^>]*)> 1:remove_underline
+addhl global/d regex <([^>]*)> 1:before_underline

test/regression/5415-final-fg-attributes-lost-to-final-attr/script

@@ -0,0 +1,2 @@
+ui_out -ignore 1
+ui_out '{ "jsonrpc": "2.0", "method": "draw", "params": [[[{ "face": { "fg": "black", "bg": "white", "underline": "magenta", "attributes": ["curly_underline"] }, "contents": "a" }, { "face": { "fg": "yellow", "bg": "default", "underline": "default", "attributes": ["final_fg"] }, "contents": "." }, { "face": { "fg": "default", "bg": "default", "underline": "default", "attributes": [] }, "contents": "<" }, { "face": { "fg": "cyan", "bg": "default", "underline": "magenta", "attributes": ["final_attr"] }, "contents": "b" }, { "face": { "fg": "yellow", "bg": "default", "underline": "default", "attributes": ["final_fg","final_attr"] }, "contents": "." }, { "face": { "fg": "cyan", "bg": "default", "underline": "magenta", "attributes": ["final_attr"] }, "contents": "c" }, { "face": { "fg": "default", "bg": "default", "underline": "default", "attributes": [] }, "contents": ">" }, { "face": { "fg": "yellow", "bg": "default", "underline": "default", "attributes": ["final_fg"] }, "contents": "." }, { "face": { "fg": "default", "bg": "default", "underline": "magenta", "attributes": ["curly_underline"] }, "contents": "d" }, { "face": { "fg": "yellow", "bg": "default", "underline": "default", "attributes": ["final_fg"] }, "contents": "¬" }], [{ "face": { "fg": "yellow", "bg": "default", "underline": "default", "attributes": ["final_fg"] }, "contents": "¬" }]], { "line": 0, "column": 0 }, { "fg": "default", "bg": "default", "underline": "default", "attributes": [] }, { "fg": "blue", "bg": "default", "underline": "default", "attributes": [] }] }'