I recently started compiling a text file showing the variations I have
used in games (both OTB and against my computer). I have standard
game records, but I want a tree showing what I tried in various
situations and what was tried against me.

I have found that a tree isn't really the exact format I want, because
I often reach the same position through a different set of opening
moves, so the "tree" combines rather than splits at that point.

It occurs to me that someone might have already designed some sort of
format for this, and I shouldn't be re-inventing the wheel. If it's
something that some program can read, all the better, but my goal is
something that can be loaded into a text editor and read by a human.

Does such a format for saving a game database exist?