Adjust `x fmt` printed output.

Currently, `x fmt` can print two lists of files.
- The untracked files that are skipped. Always done if within a git
  repo.
- The modified files that are formatted.

But if you run with `--all` (or with `GITHUB_ACTIONS=true`) it doesn't
print anything about which files are formatted.

This commit increases consistency.
- The formatted/checked files are now always printed. And it makes it clear why
  a file was formatted, e.g. with "modified".
- It uses the same code for both untracked files and formatted/checked
  files. This means that now if there are a lot of untracked files just
  the number will be printed, which is like the old behaviour for
  modified files.

Example output:
```
fmt: skipped 31 untracked files
fmt: formatted modified file compiler/rustc_mir_transform/src/instsimplify.rs
fmt: formatted modified file compiler/rustc_mir_transform/src/validate.rs
fmt: formatted modified file library/core/src/ptr/metadata.rs
fmt: formatted modified file src/bootstrap/src/core/build_steps/format.rs
```
or (with `--all`):
```
fmt: checked 3148 files
```

0 files changed, 0 insertions, 0 deletions