The new DVCS support in the diff viewer had some apparent off-by-one

issues when attempting to select ranges of commits. Performing a diff

using two commits in a range would often cause the first commit's

changes to be excluded. This was always the case when involving the

first commit in the series, and also when involving the first commit

touching a given file. Further, viewing a subsequent commit by itself

could cause some changes from the parent commit to be included.
This was due to a few problems.
First, some code cleanup changed the behavior of a loop used to

calculate a base FileDiff, and this meant that the wrong base was being

used. It would prioritize the wrong end of the list of ancestors. The

original, unbroken code has been restored, and in order to help isolate

this logic and keep it working, it's been moved to

FileDiff.get_base_filediff. A unit test was added to ensure it

returned the correct results.
Second, DiffChunkGenerator wasn't properly handling the cases of

FileDiffs belonging to commits. When a base FileDiff was provided, its

original file content was being used as the "old" code in the diff

viewer, with its own content becoming part of the "new" code. Since this

is meant to be the base (the parent of what you're seeing), this meant

that too much content was leaking. The solution was to patch the old

content with the base commit's modifications.
Third, and related, FileDiffs belonging to commits weren't working when

a base FileDiff couldn't be provided. This would be the case if viewing

any range involving commit #1, or any commit that initially touches a

file in the series. The base commit and its ancestors didn't modify this

file, and thus a base FileDiff couldn't be provided, and the "old"

content would therefore stay as the tip FileDiff's original content

instead of the first modified version of the file in the range of

commits. We now check for this case, find the first ancestor (which will

be newer than the base commit), and use its original content as the

"old".
Finally, we were relying on dictionary order when computing a list of

commits for the review request page. This wasn't safe and led to some

JavaScript problems. We now sort these before passing them to the page.

Unit tests passed.
Checked this against a few review requests I put together that

reproduced all these problems. Everything appears to work exactly as

expected now.