[QUESTION] ref-filter: how to support atom like %an

[ZheNing Hu <adlternative@gmail.com>]

My current thinking is this:

First add a new member `short_name` in `struct valid_atom`,
Then we can share a valid_atom item for the long name of
an atom such as `%(authoremail)` and the short name of
the atom `%ae` (%aE is not considered at the time).
Something like:

[ATOM_AUTHOREMAIL] = { "ae", "authoremail", SOURCE_OBJ, FIELD_STR,
person_email_atom_parser },

Then we need to do some work on atoms parsing:
In the original design of `parse_ref_filter_atom()`, it relied on
searching for `%(` and `)` to find an atom. Now we have to
change the way of finding atoms: Currently I just replace
`find_next()` to find a single `%`. If there is `(` after `%`,
we can keep the original long name atoms parsing method,
otherwise we are dealing with the short name atom,
we need to pass the interval between the current `%`
and the next single `%` to `parse_ref_filter_atom()`.
Then we are in `parse_ref_filter_atom()`, we need to
consider these situations:

1. %(authoremail:trim)
2. %(authoremail)
3. %ae
4. %ae:trim
5. %aeabc:trim
6. %ae:trimabcd...

As we can see, the situation is starting to get complicated.
The more critical point is as in case 6: We cannot determine
the end of a short name atom because it does not have an
explicit `)` end.
In this way, many of the `*_atom_parser` functions in
`ref-filter.c` will not work, because most of them use `strcmp()`
to check the attribute of an atom. If we want to support the
short name `%ae`, that would require a large-scale rewrite.

Any good ideas, friends?

--
ZheNing Hu