Provide a way to merge other log file into current one

Applications using the library sometime need to merge other logs into the current one (for example, the collected output of a subprocess run).

The library should provide a method like `merge_logs_from` to merge lines from other file into the current one.

The payload names no implementation files or tests. Start by locating the library's current log-file handling and the code that writes log lines; determine how another file should be read and merged, then verify that the current log contains the other file's lines in the intended order.

Written by the indexing model from the issue text.