Situation: Some commit (on master, but not necessarily head of master) has broken things, but it's a big commit and it's not clear what part broke things.

```sh
% git checkout master
% git checkout -b bisect-branch
% git revert <offending commit>
```
(test here to make sure reverting fixed your problem)
```sh
% git bisect start
% git bisect good
% while [ "`git diff master`" != "" ]; do echo -e "y\nq\n" | git checkout -p master; git commit -m 'partial undo'; done
```
(test here to make sure we've recreated your problem)
```sh
% git bisect bad
```

Now just follow the bisect.

The `while` line has broken your commit up into small chunks, one commit per chunk.  If a chunk has actually broken syntax (so you can't test either way), you can `git bisect skip`.

You can also `s/master/any-reference-branch-or-tag-or-sha/g`.  And you can use `s\ny\nq\n` if you want even smaller chunks (but more risk of syntax errors).