Backport #17406.
Closes#17378
Both errors from #17378 were caused by #15175.
Problem 1 (error with added file):
`ToUTF8WithFallbackReader` creates a `MultiReader` from a `byte[2048]` and the remaining reader. `CreateReaderAndGuessDelimiter` tries to read 10000 bytes from this reader but only gets 2048 because that's the first reader in the `MultiReader`. Then the `if size < 1e4` thinks the input is at EOF and just returns that.
Problem 2 (error with changed file):
The blob reader gets defer closed. That was fine because the old version reads the whole file into memory. Now with the streaming version the close needs to defer after the method.
Co-authored-by: zeripath <art27@cantab.net>
Implements request #14320 The rendering of CSV files does match the diff style.
* Moved CSV logic into base package.
* Added method to create a tabular diff.
* Added CSV compare context.
* Added CSV diff template.
* Use new table style in CSV markup.
* Added file size limit for CSV rendering.
* Display CSV parser errors in diff.
* Lazy read single file.
* Lazy read rows for full diff.
* Added unit tests for various CSV changes.
