firebase.json: functions.ignore not including deeper directories

### [REQUIRED] Environment info

**firebase-tools:** `13.29.1`

**Platform:** `macOS`

### [REQUIRED] Test case

See simple setup here https://github.com/rhodgkins/firebase-tools-functions-ignore

I have a directory named `deployment` at the root inside the `functions.source` directory, and also another directory `deployment` in the `foobar` directory at the root. I want the root `deployment` directory to not be uploaded with the functions.

- `functions/`
- `deployment/` - ignore this whole directory
- `foobar/`
- `deployment/` - want this directory (and everything inside it) included

### [REQUIRED] Steps to reproduce

Deploy the functions:

`firebase --project=XXX deploy --only=functions`

Found this due to having a deployment directory (with some config files not related to the function runtime execution) and another sub directory called deployment which was named after some function functionality. Couldn't work out why the directory wasn't being uploaded.
I've tried using `deployment/*` in the `ignore` instead but then the root `deployment` directory gets uploaded.

I think it's mainly due to how paths are filtered here:
https://github.com/firebase/firebase-tools/blob/e145360f2d47f50be21d167d50ddf6f124a54ac4/src/fsAsync.ts#L23

It shouldn't include base `sourceDir` and then the `minimatch` filters will work correctly with `deployment/*`.

Going to attempt a fix, or at least start off with a PR with a failing test case for `readdirRecursive`

### [REQUIRED] Expected behavior

For `functions/foobar/deployment` directory to be uploaded, but `functions/deployment` to not be included.

### [REQUIRED] Actual behavior

`functions/foobar/deployment` and everything in it, is excluded.