Fossil SCM

When a ticket is created or modified Fossil extends BACKLINK table with the links extracted from the J-cards of the [ticket change artifact] (/doc/2022-05-18/www/fileformat.wiki#tktchng). That extraction requires the knowledge of which markup is used.

Previously Fossil derived this information solely from the value of J-card [named "mimetype"]¹, which was considered only if a regular "mimetype" column was defined in TICKET or TICKETCHNG table.

This branch adds the ability to specify mimetypes using GENERATED "mimetype" columns.
It implements the following rules:

1. If either TICKET or TICKETCHNG table has a regular (not generated) "mimetype" column, then traditional processing is applied (as schetched in the beginning).

2. Otherwise: if just one of the above tables has generated "mimetype" column then the generated value in the corresponding row is used.

3. Otherwise: if just one of two generated values is not NULL then that value is used for all fields.

4. Otherwise (when both generated values are not NULL):

   • all fields that are defined in the TICKETCHNG table are processed using mimetype from the TICKETCHNG table;
   • the rest fields (that are specific to TICKET table) are processed using mimetype from the TICKET table.

If a NULL value has to be interpreted as mimetype it works as Fossil-Wiki. That also applies to the case when "mimetype" column is missing in both TICKET and TICKETCHNG.

See forum thread 40c1208a0f84 for rationale and discussion.