The diff comment serialization worked very differently from file

attachment and screenshot comment serialization. This change updates

that to match.
The structure used to be:

[
    {
        "linenum": X,
        "num_lines": Y,
        "comments": [
            ... comments ...
        ]
    },
    ...
]


Now it's:

{
    "X-Y": [
        ... comments ...
    ]
}


The object keys in the new format are used to group all comments on the

same block together, but are otherwise opaque. Instead, like we do for

file attachments, we'll pick the line and num_lines values out of

the first comment in the list.
It's arguable that the diff format was better than the one used for file

attachments, since the line number and number of lines for each block

really should be serialized separately from the individual comments.

Unfortunately, the file attachment format isn't easily changeable because

extensions that add Review UIs exist in the wild, and we need to

maintain compatibility there.

Loaded a diff with existing comments and saw that they all appeared

  correctly.
Ran python tests.
Ran js-tests.