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

[Yuya Nishihara]

>   @yuja  I am adding an --unmodified flag to change to change the mode to grep on the unmodified files as well.
>   What I am trying to do is ::
>   
>     def fns_generator():
>         if --unmodified:
>             for f in ctx:
>                 for f in ctx:
>                     if match(f):
>                         yield f
>         else :
>             for f in ctx.files():
>                 for f in ctx:
>                     if match(f):
>                         yield f

Nit: `if` can be narrowed to `ctx`/`ctx.files()`.

```
if --unmodified:
    fiter = iter(ctx)
else:
    fiter = ctx.files()
```

>   Currently, this would work for a single revision, so I am modifying the `prep` function in grep.
>   Shall I continue and send the patch ?

Seems fine. It should be generally encouraged to send patches than discussing
for long without a working code.

> but @yuja has asked me to break this patch into three patches. So I guess
> this is not going to be merged anyway.

To be clear, I suggested to split patches because the original code seemed
to add a single knob to switch two more features at once.