Use correct filename in auto commands.

This problem was revealed when using BufExplorer:

- Open vim with two files: `$ vim a.txt b.txt`
- Type `\\` to open BufExplorer.
- Press <enter> on `b.txt`.
- Vim shows `b.txt` as expected.
- Type `\\` to open BufExplorer.
- Press <enter> on `a.txt`.
- Expected: Vim shows `a.txt`.
  Actual Vim shows `a.txt`, `b.txt`, `a.txt`.

I.e. the first invocation was fine but all subsequent invocations
rapidly flicked through <new file>, <old file>, <new file>.