obsfate: fix obsfate_printer with empty date list When the list of dates is empty, `min` and `max` would raises a ValueError. Protect against this case by checking that the date list is not empty. I didn't add a test because I couldn't find a reproducing test case.

config: gather allowdivergence under the evolution namespace Grouping all evolution related-config under the experimental.evolution namespace would helps the future migration outside [experimental]. Differential Revision: https://phab.mercurial-scm.org/D1155