Add --output=json support to revs command

[AndreyVMarkelov]

Summary

• Migrate revs to emit file revisions as structured JSON with {"input": {...}, "entries": [...]}
• revsInput includes file path and display options (long, time, time_format)
• Each entry contains full file metadata (rev, size, timestamps)
• Text mode behavior unchanged

Test plan

• go vet ./... clean
• golangci-lint run ./... clean
• All tests pass (go test ./... -count=1)
• Manual: dbxcli revs --output=json /file.txt emits JSON with revision entries
• Manual: dbxcli revs -l /file.txt text output unchanged