bug#61281: “`(a \, b)” equals to “`(a . , b)”

[Drew Adams]

> `(a  ,@ foo) ; ==> (a 4 5)
> `(a \,@foo)  ; ==> (a \,@ foo) - good
> `(a \,@ foo) ; ==> (a \,@ foo) - good
> 
> `(a  , foo) ; ==> (a 4 5)
> `(a \,foo)  ; ==> (a \,foo) - good
> `(a \, foo) ; ==> (a 4 5)   - bad, the bug case

Sorry for the typos in the comments.
This is what it is:

1. `(a  ,@ foo) ; ==> (a 4 5)
2. `(a \,@foo)  ; ==> (a \,@foo)  - good
3. `(a \,@ foo) ; ==> (a \,@ foo) - good

4. `(a  , foo) ; ==> (a (4 5))
5. `(a \,foo)  ; ==> (a \,foo) - good
6. `(a \, foo) ; ==> (a 4 5)   - bad, bug case

Note that #6 is not the same as #4.  It's not
just that the backslash made no difference.
#6 is the same as #1: the backslash made the
comma act like ,@.