completion: complete more values of more 'color.*' configuration variables

completion: complete more values of more 'color.*' configuration variables

Most 'color.*' configuration variables, with the sole exception of
'color.pager', accept the same set of values, but our completion
script recognizes only about half of them.  We could explicitly add
all those missing variables, but let's try to reduce future
maintenance burden, and use the catch-all 'color.*' pattern instead,
so this list won't get out of sync when a similar new configuration
variable accepting the same values is introduced [1].

Furthermore, their documentation explicitly mentions that they all
accept the standard boolean values 'false' and 'true' as well, so list
these, too, among the possible values.

[1] OTOH, there will be a maintenance burden if ever a new
    'color.something' is introduced which doesn't accept the same set
    of values.  We'll see which one happens first...

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>