Strictly speaking, one can never be 100% sure that a commit doesn't break a program.
Even with all sorts of testing possible (unit testing, integration, component, system, manual, UI, fuzz, security, penetration .. you name it). This is due to a Halting problem. A relevant extract from the Wikipedia follows below:
In computability theory, the halting problem can be stated as follows: "Given a description of an arbitrary computer program, decide whether the program finishes running or continues to run forever". This is equivalent to the problem of deciding, given a program and an input, whether the program will eventually halt when run with that input, or will run forever.
Alan Turing proved in 1936 that a general algorithm to solve the halting problem for all possible program-input pairs cannot exist. A key part of the proof was a mathematical definition of a computer and program, which became known as a Turing machine; the halting problem is undecidable over Turing machines.
If your PM cares about value and stable predictable delivery, you can perhaps convince him to have a look at SCRUM framework.