Do not import vars that are equivalent to a universal exported var

Universal exported variables (created by `set -xU`) used to show up
both as universal and global variable in child instances of fish.

As a result, when changing an exported universal variable, the
new value would only be visible after a new login (or deleting the
variable from global scope in each fish instance).

Additionally, something like `set -xU EDITOR vim -g` would be imported
into the global scope as a single word resulting in failures to
execute $EDITOR in fish.

We cannot simply give precedence to universal variables, because
another process might have exported the same variable.  Instead, we
only skip importing a variable when it is equivalent to an exported
universal variable with the same name.  We compare their values after
joining with spaces, hence skipping those imports does not change the
environment fish passes to its children. Only the representation in
fish is changed from `"vim -g"` to `vim -g`.

Closes #5258.
This eliminates the issue #5348 for universal variables.

tests/checks/set.fish

@@ -462,4 +462,16 @@ begin
     # CHECK: inner
 end
 
+# Skip importing universal variables (#5258)
+while set -q EDITOR; set -e EDITOR; end
+set -Ux EDITOR emacs -nw
+# CHECK: $EDITOR: not set in global scope
+# CHECK: $EDITOR: set in universal scope, exported, with 2 elements
+$FISH -c 'set -S EDITOR' | string match -r -e 'global|universal'
+
+# When the variable has been changed outside of fish we accept it.
+# CHECK: $EDITOR: set in global scope, exported, with 1 elements
+# CHECK: $EDITOR: set in universal scope, exported, with 2 elements
+sh -c "EDITOR='vim -g' $FISH -c "'\'set -S EDITOR\'' | string match -r -e 'global|universal'
+
 true