bug#16145: ERC disregards data to be logged when reconnecting

[Ivan Shmakov]

        Upon reconnecting to the server, erc-connect-pre-hook is run,
        which, by default, results in erc-initialize-log-marker being
        called, and the erc-last-saved-position marker reinitialized.

        The problem is that no calls to erc-save-buffer-in-logs are made
        when the connection is closed, resulting in the entire previous
        ERC session silently disregarded, and never saved in the logs.

        Presuming that erc-initialize-log-marker was really meant to
        only initialize erc-last-saved-position when there’s none, my
        suggestion would be for it to be amended with an appropriate
        ‘unless’ guard.

        Alternatively (or perhaps complementarily), a call to
        erc-log-save-all-buffers may be added to the
        erc-disconnected-hook’s default value.  (Please note that this
        alone, however, may be less reliable a measure.)

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FSF associate member #7257