D3826: grep: change default behaviour of grep

[Yuya Nishihara]

Queued, thanks.

>    $ hg up -q null
> -  $ hg grep -f port
> -  [1]
> +  $ hg grep -r tip:0 -f port

I've changed this to `-r 'reverse(:.)'` since it was the default for `-f`.

> -    By default, grep prints the most recent revision number for each
> -    file in which it finds a match. To get it to print every revision
> -    that contains a change in match status ("-" for a match that becomes
> -    a non-match, or "+" for a non-match that becomes a match), use the
> -    --diff flag.
> +    By default, grep searches the expression on the working directory
> +    , to search history and show the most recent revision number for each
> +    file in which it finds a match, use `hg grep -r tip:0`.
> +    To get it to print every revision that contains a change in match status
> +    ("-" for a match that becomes a non-match, or "+" for a non-match that becomes
> +    a match), use the --diff flag.

Perhaps this needs an English tweak, but I'm not a right person. So I only
fixed trivial stuffs.