Previously, the logic for when to add a date row to an edit history
entry was checking against the date of the original message (which is
always the first entry in the message history), not the date of the
previous edit. This caused every edit not made on the date of the
original message to show a date row, even if it wasn't the first edit
on that date.
This commit fixes that bug by updating prev_timestamp after processing
each message history entry, whereas before it was only updated after
processing the first one — the original message.
This bug has existed since June 2017, when
84e5fe733c changed how date rows worked;
from only showing one at the top labeled "Earliest" to each entry
having a possibilty of showing one.
Previously it was impossible for a topic-only edit to show a date row
in any circumstance; the code that handles topic-only edits didn't
even attempt to set show_date_row, the flag that determines whether a
date row should be rendered. Now a topic-only edit will show a date row
in the same circumstances as any other edit[1].
This bug has existed since March 2019, when rendering of topic-only
edits was first added in 38be5ea743.
[1] Currently, "the same circumstances as any other edit" means
there'll be a date row on the original message, and then on every edit
not made on the same date as the original message, even if it was't
the first edit on the date it was made. This is a bug that will be
fixed in a following commit. This commit is being made first since
it's fixing a lack-of-information bug, whereas the other bug is a
somewhat less important repeating-information bug.
