The New Review Request page's commit list loads an initial page of

commits and then only loads new pages of commits when scrolling to the

end of the list. However, if the window is tall enough, that initial

list of commits won't be enough to trigger a scrollbar, preventing any

more commits from loading.
We now progressively load pages of commits until we know we've filled up

the visible list and triggered a scrollbar. The logic is the same as the

one we used for determining when to load a new page of commits when

scrolling, but is now called on initial page load and can repeatedly

call itself until the visible list is full.
As part of this, RB.RepositoryCommits.fetchNext() now takes options

for notifying on success or error, which is used by

RB.CommitsListView both for the progressive loading and for error

reporting if progressive loading fails (which we didn't notify about

before).

Unit tests passed.
Tested by shrinking the font to an ultra-tiny size with a full-screen

window. Without the patch, the commits wouldn't fill up the available

space, and I had no way of loading new commits. After the patch, the

commits filled up the entire available width, and I could scroll to

load more.