codeSplitters branches: delete a file on a branch (and carry upstream deletions and binary files into mirror branches) #440

Split out of #414 (the rest of it is fixed by the PR that closes #414).

codeSplitters branches (src/branches.ts) are Durable Object copies of the files they touch, and a copy can only be edited or new: there's no way to delete a file on a branch and have the merge delete it from main. So:

• Opening an upstream branch on a mirror (POST /api/repos/:o/:r/mirror/branches), and a branch catching up with its upstream branch (pull, or each sync), list the files upstream deleted as skipped.
• Binary files are skipped the same way, since a branch copy is lines of text.
• Pushing a branch upstream can't delete a file either.

Needs: a "deleted" mark on branch_files (or a tombstone copy), the review and merge showing and applying it, and take/pushBranch in src/mirror.ts using it. Binary files would need a blob-backed copy, which might be its own issue.