Before this change:
We requests /diff for file1
We processed file1
We requests /diff for file2
...
After this change
We prefetch /diff for each file
We use pretech of request /diff for file1
We processed file1
...
This is saving at least 100ms per file, for 10 files ~1second.
Change-Id: I60c928852f2b7376c724b1c996966ac8f242cc8a