R1:0759b84d77c0

Improve construction of commit queries from blame lookups

Summary:
Ref T2450. File blame tends to have the same commit a lot of times, and we don't do lookups like this efficiently right now.

In particular, for a file like `__phutil_library_map__.php`, we would issue a query with ~9,000 clauses like this:

```
(repositoryID = 1 AND commitIdentifier LIKE "XYZ%")
```

...but only a few hundred of those identifiers were unique. Instead, issue only one clause per unique identifier.

MySQL also seems to do a…