We also renamed `filter` (as a name of a parameter) to `predicate` following the naming suggestion in code review. It's better!
Since it's not part of an attrset, the name can change with no impact to semantics, since it can't be observed with `builtins.functionArgs`.
```
$ nix-repl
Nix 2.21.0
Type :? for help.
nix-repl> f = x: y: z: (x + y + z)
nix-repl> builtins.functionArgs f
{ }
nix-repl> :doc builtins.functionArgs
Synopsis: builtins.functionArgs f
Return a set containing the names of the formal arguments expected by the function f. The value of each attribute is a Boolean denoting whether the corresponding argument has a default value. For instance, functionArgs ({ x, y ?
123}: ...) = { x = false; y = true; }.
"Formal argument" here refers to the attributes pattern-matched by the function. Plain lambdas are not included, e.g. functionArgs (x: ...) = { }.
```
There are a few operations in this library that naively runs on every
iteration while they could be cached.
For a simple test repository with a small number of files and ~1000
gitignore patterns this brings memory usage down from ~233M to ~157M
and wall time from 2.6s down to 0.78s.
This should scale similarly with the number of files in a repository.
