journalctl: fix when --grep is used with --follow

Follow-up for #25147 (db46919)

--follow sets arg_lines to 10, which breaks
--grep as the latter implies --reverse.
So let's not set --reverse if --follow is used.

(cherry picked from commit c673fd5)

Should fix https://bugzilla.redhat.com/show_bug.cgi?id=2215304.

Author: YHNdnzj

src/journal/journalctl.c

@@ -1089,8 +1089,10 @@ static int parse_argv(int argc, char *argv[]) {
                         return r;
 
                 /* When --grep is used along with --lines, we don't know how many lines we can print.
-                 * So we search backwards and count until enough lines have been printed or we hit the head. */
-                if (arg_lines >= 0)
+                 * So we search backwards and count until enough lines have been printed or we hit the head.
+                 * An exception is that --follow might set arg_lines, so let's not imply --reverse
+                 * if that is specified. */
+                if (arg_lines >= 0 && !arg_follow)
                         arg_reverse = true;
         }