Tcl, Fossil and SQLite share an interesting history. SQLite started as a Tcl extension and the SQLite maintain Dr Richard Hipp is part of the Tcl Core Team as well. Fossil started out as a version control for SQLite development by Dr Richard Hipp as well, and might have influenced the Tcl development as well.
I did (along with the help of several other people finding, building, and testing tools) the migration of Tcl from CVS to Fossil during the great Sourceforge outage. We looked at many options for migrating to such as Darcs, Git, Bzr, and Mercurial -- Larry McVoy (bitkeeper author and Tcl contributor and supporter, also on HN) offered Bitkeeper support as well. The core team liked Fossil the best at the time though -- some people do not view this as the right decision to this day.
Source: https://www.sqlite.org/tclsqlite.html https://wiki.tcl-lang.org/page/D.+Richard+Hipp