BITCOUNT 'end' as optional argument for consistency with BITPOS

[LiiNen]

I have found that something weird between BITPOS and BITCOUNT:

BITPOS key bit [start [end [BYTE | BIT]]]
BITCOUNT key [start end [BYTE | BIT]]

I think this might hurt consistency and can cause confusion in terms of usability.
The results below are the asis unstable build:

> get TEST:ABCD
"ABCD"
> BITPOS TEST:ABCD 1 0 -1
(integer) 1
> BITCOUNT TEST:ABCD 0 -1
(integer) 9
> BITPOS TEST:ABCD 1 0
(integer) 1
> BITCOUNT TEST:ABCD 0
(error) ERR syntax error

And these PR contains fixing of this issue:

> GET TEST:ABCD
"ABCD"
> BITPOS TEST:ABCD 1 0 -1
(integer) 1
> BITCOUNT TEST:ABCD 0 -1
(integer) 9
> BITPOS TEST:ABCD 1 0
(integer) 1
> BITCOUNT TEST:ABCD 0
(integer) 9

Syntax hint

# ASIS 
> BITCOUNT key [start end [BYTE|BIT]]
# TOBE
> BITCOUNT key [start [end [BYTE|BIT]]]

I think this minor fixes make redis more consistent, and this may not cause any breaks.

[CLAassistant]

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