Just splitting this out of T13105 since I'm currently dragging a nontrivial TODO list through the comments.

• Degrading and error messages when blaming large files.
• "Skip Past This Commit" doesn't retain line numbers.
• When we know the commit created the file, disable the "skip past this commit" stuff.
• Symbol highlighting has some glitchiness in Safari, e.g. typehints.
• Lint messages.
• General cleanup of old behaviors? (I think I got all of these?)
• Paste embeds, like {P2097}, have a little jank to them that should get cleaned up here since this is probably a closer fit than T13088 (they get a mention there, too).
• Fatal in blame (see below).
• See D19391. The DiffusionController->loadBlame() method can currently return identifiers like ^abc123 under Git for lines introduced in the initial commit. This is a "boundary commit" marker and means "blame can't go back any further". It's relevant for shallow clones or git blame --since but we should almost certainly just strip it in Diffusion. Today, it makes it up to the caller and can cause bad behavior. D19391 makes one class of this behavior smoother.