Cursor should not be able to be placed after the last line of the diff

[smashwilson]

In our FilePatchItem editors, you can click beyond the last line of the diff content, which is awkward.

Originally reported by @Arcanemagus in #1512 (comment).

[Arcanemagus]

Copying here so you don't have to expand all the collapsed content to see it:

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This is relatively minor, but I just noticed you can place the cursor after the last line of the diff:

Using this branch directly at 59b4b45, on Atom v1.33.0-nightly9.

[annthurium]

Once diffs are editable, it seems like we should allow you to put the cursor after the last line in case you want to extend the file, yes?

[Arcanemagus]

That isn't the behavior when editing a file normally, so that doesn't make too much sense to me at least.

There is a setting that allows you to scroll past the end of the file contents, but you can't place a cursor past the end of the contents. When we get to editable diffs they should match the regular editing experience as closely as possible.