Make abbreviation completions shadow commands

As reported in
https://matrix.to/#/!YLTeaulxSDauOOxBoR:matrix.org/$yD_Lutaftf6ytk617kjw5vC-k_OgHRQxIiSRv89uBMI
it's weird that command name completions shadow abbreviation name
completions, given that the abbreviation will the one that will be
executed in the typical cases. Let's put abbreviation completions first;
unique_completions_retaining_order() will take care of removing any command
completions we add later.

tests/checks/complete.fish

@@ -629,3 +629,7 @@ complete -C'testcommand '
 # CHECK: check{{\t}}Check the frobnicator
 # CHECK: search{{\t}}Search for frobs
 # CHECK: show{{\t}}Show all frobs
+
+abbr cat cat
+complete -C ca | string match -r '^cat(?:\t.*)?$'
+# CHECK: cat{{\t}}Abbreviation: cat