How do I use log to find that src/modules/_math was originally called src/modules/math?
Try: git blame src/modules/_math/math.c
Well, ok, I guess I underspecified the question. Yes, you can see from this output that the file has at some point had this name (but you can't know that it's the _original_ name, it might have had another name before that). What I mean was, how can I find the actual rename operation, which should contain the time of the rename, the old pathname and the new pathname.
(The blame trick would not have worked if all the lines had (eventually) changed after the rename.)