ui-blame: add a link to the parent commit in blame

When walking through the history, it is useful to quickly see the same
file at the previous revision, so add a link to do this.

It would be nice to link to the correct line with an additional
fragment, but this requires significantly more work so it can be done as
an enhancement later.  (ent->s_lno is mostly the right thing, but it is
the line number in the post-image of the target commit whereas the link
is to the parent of that commit, i.e. the pre-image of the target.)

Suggested-by: Alejandro Colomar <alx.manpages@gmail.com>
Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>