lines.Send() only support byte delimiter

During the work on pagination for FindAllLocalBranches in e3b1d6e5 I ran into buggy behaviour when multi line delimiters were used in lines.Send(). One example: if a delimiter is twice the same character, it would unsuccessfully detect the last delimiter and thus send invalid results to a client.

Luckily there's no call site that actually uses multi byte delimiters, which allows this change to remove support for it.

Closes: #2846 (closed)