Clearer cloud file sync errors for large nested ignore files and changing worktrees

Two cloud file sync problems now get their own handling:

• File sync is not offered when the nested ignore files (such as .gitignore files in subfolders, which list files git should skip) are too large
• Uploading from a linked worktree (an extra checkout that shares one repository) whose git files change while being read now fails with a specific message suggesting a retry, instead of a generic error about not being able to read where git keeps the checkout