Fossil 2.16 features and their implications to PDQ

[pcmacdon]

Fossil 2.16 which was released in July 2021, contains new features that have important implications for PDQ.

The first of these is the fix to this bug:

$:~/tmp/pdq_4$ fossil.2.15 open pdq.fossil 
directory /home/user/tmp/pdq_4 is not empty
use the -f or --force option to override

The second is the addition of the "fossil patch" command.

Because of this, going forward PDQ will likely require fossil 2.16.