The fish shell comes with builtin completions. For example, git
completion supports context-aware completion of things like commit
hashes, branch names, sub-commands, etc.
Until fish 4.2, builtin completions were explicitly loaded from
`share/fish/completions`, however that is now deprecated and disabled.
In effect, this means generating manpage-based completion will shadow
and disable builtin completion.
Avoid that, by only generating completion when fish does not have
builtin support for the command.
(cherry picked from commit 23f2ba7ae0)
3fdd076e08