Nathan Lanza 27b52acee7
Only attempt to use dirvish if b:dirvish exists
This was causing persistent errors when you run, for example,

```
ls | nvim -c 'setf dirvish'
```

as dirvish will not yet have set that local variable yet airline will
attempt to use it.
2020-09-15 12:30:27 +02:00
..
2020-07-20 21:41:59 +10:00
2020-08-15 02:52:48 +04:00
2020-01-06 11:22:49 +09:00
2020-09-08 01:26:24 +09:00
2020-01-06 11:22:49 +09:00
2020-01-06 11:22:49 +09:00