The problem is that incompatible (git and release (0.15.1)) versions use the
same directory to store savegames and configs, did I get this right?
Maybe, inside the config dir ~/.freedroid_rpg; we could make one folder for each
release.

~/.freedroid_rpg/git/
~/.freedroid_rpg/0.15.1/
~/.freedroid_rpg/0.15/

and let the game chose the right path according to its version.

However, what if savegames of latest release and git are actually still compatible?
Perhaps it is possible to silently check if the savegames are still compatible
and only display those which are, or just make them loadable (even if there are
errors) but save them into the newer (git in this case) version-directory.