While reviewing cpuprofiles of #10985, I noticed a bunch of slice-grows within this function.
Instead of `io.ReadAll` (which does not know the size in advance), use `fs.ReadFile`, which will perform correctly-sized allocations.
### Tests
This function is already covered by tests in `modules/assetfs/layered_test.go`
### Documentation
- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.
### Release notes
- [ ] This change will be noticed by a Forgejo user or admin (feature, bug fix, performance, etc.). I suggest to include a release note for this change.
- [x] This change is not visible to a Forgejo user or admin (refactor, dependency upgrade, etc.). I think there is no need to add a release note for this change.
*The decision if the pull request will be shown in the release notes is up to the mergers / release team.*
The content of the `release-notes/<pull request number>.md` file will serve as the basis for the release notes. If the file does not exist, the title of the pull request will be used instead.
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/10987
Reviewed-by: Michael Kriese <michael.kriese@gmx.de>
Reviewed-by: Mathieu Fenniak <mfenniak@noreply.codeberg.org>
Co-authored-by: oliverpool <git@olivier.pfad.fr>
Co-committed-by: oliverpool <git@olivier.pfad.fr>
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