This allows making something like
```fish
abbr --add gc --position anywhere --command git back 'reset --hard
HEAD^'
```
to expand "gc" to "reset --hard HEAD^", but only if the command is
git (including "command git gc" or "and git gc").
Fixes#9411
The regex struct is pretty large at 560 bytes, with the entire
Abbreviation being 664 bytes.
If it's an "Option<Regex>", any abbr gets to pay the price. Boxing it
means abbrs without a regex are over 500 bytes smaller.
