Now it's based on calling fillMore rather than an illegal aliased stream
into the Reader buffer.
This commit also includes a disambiguation block inspired by #25162. If
`StreamTooLong` was added to `RebaseError` then this logic could be
replaced by removing the exit condition from the while loop. That error
code would represent when `buffer` capacity is too small for an
operation, replacing the current use of asserts.
6a25bfc475