Numbers referring to unset $e variables should not be removed

[HaleTom]

git allows referring to a commit by the first 4 hex digits.

When these 4 digits are numbers, exec_scmb_expand_args will remove them.

% ge echo foo
foo
% ge echo 9999

%

Also, null strings should not be removed:

% shell-quote ''
''
% shell-quote '' a\ b
'' 'a b'
% ge shell-quote '' a\ b
'a b'

[ghthor]

Yeah, I agree. `exec_scmb_expand_args` should only consume and resolve digits with matches $e values.

[HaleTom]

Related to #84