D2938: grep: make grep search on working directory by default

[Yuya Nishihara]

>   So I was working on it, but the way I tried to correct "wdir()" is by handling it as a special case, so introducing an "if" block again similar to this.
>   But you were asking to eliminate the if/else block altogether, I wonder if there is way ?

IIRC, my point was to avoid adding *big* if/else branch which will be likely
to introduce untested code paths.

I don't think there would be a way to go without if/else or try/except as
long as we use the low-level filelog API for performance reasons. Perhaps,
we'll have to catch WdirUnsupported error to fall back to the filectx API.

```
try:
    flog.read(fnode)
except error.WdirUnsupported:
    ctx[fn].data()
```