textiter: small optimization for find_by_log_attrs()

Use gtk_text_iter_set_line_offset (&tmp_iter, 0) instead of
gtk_text_iter_get_line(). The difference should not be big. In the first
case the line doesn't need to be traversed thanks to the offset 0. For
get_line(), the btree must be traversed.

A temporary iter is needed to not break the behavior. But the behavior
is quite strange, the function works directly on the iter passed as an
argument to the function, even if the function returns FALSE (not
found). So maybe a later commit will fix this strange behavior.

https://bugzilla.gnome.org/show_bug.cgi?id=629129