journalctl: print all possible lines immediately with --follow + --since

journalctl: print all possible lines immediately with --follow + --since

When I tryed to run journalctl with --follow and --since arguments it
behaved very strangely.
First It prints logs from what I specified in --since argument, then
printed 10 lines (as is default in --follow) and when app put something
new in to log journalctl printed everithing from the last printed line.

How to reproduce:
1. run: journalctl -m --since 14:00 --follow
Then you'll see 10 lines of logs since 14:00. After that wait until some
app add something in the journal or just run `systemd-cat echo test`
2. After that journalctl will print every single line since 14:00 and will
follow as expected.

As long as --since and --follow will eventually print all relevant
lines, I seen no reason why not to print them right away and not after
first new message in journal.

Relevant bugzillas:
        https://bugs.freedesktop.org/show_bug.cgi?id=71546
        https://bugs.freedesktop.org/show_bug.cgi?id=64291