In T4387, we moved some `hg grep` calls to `hg locate`. However, `hg locate` is deprecated as of Merucrial 3.2 (2014-11-01, roughly a month after we moved to use it).
The replacement is `hg files`, which is new as of 3.2.
So we need to:
- Use `hg locate` on `hg` < 3.2.
- Use `hg files` on `hg` >= 3.2
- (`hg files` may have slightly different syntax?)