Detect trimmed changelogs and pick online instead

(Modified the last commit from !281 slightly to make it more agreeable for bookworm without the extra fluff. I presume 281 can be closed as-is and I will see about bringing parts of it to trixie)

We only check the start of these lines to avoid hard coding the exact command and we pick 150 as maximum line length as the longest package name on my system is apparently 75 characters long. We could choose longer or shorter without much issue as over-length just means we mishandle the rest of the line as a new line and it should be really unlikely that a) lines are that long in this file and b) that such long lines contain one of our trigger sequences – but even if, all we do is start a download of an online file. Could be worse.

This auto-detection can be avoided by setting Acquire::Changelogs::AlwaysOnline (or Origin specific sub options) to "true" if you always want the changelog from an online source. The reverse – setting it to "false" in the hope it would not get the changelog from an online source – was not and is still not possible.

Closes: #1024457