The p=X and ft=Y query parameters on /timeline cause a timeline that shows
all ancestors of X "back to" Y.  We need a corresponding query parameter
set to show descendants of X up to Y.

A related request is to allow both bt=Y1 and ft=Y1 on a pd=X request.

The argument to the ft= query parameter should be resolved as the earliest
matching tag in the descendant set of X, not as the latest chronological
match.  See [ticket ed62a4d35332595a](/info/ed62a4d35332595a).

## Use case

If a bisect lands on version X, this feature could be used as
"`p=X&ft=release`" to figure out the first release in which the
problem appears.

Need d=X, ft=Y on /timeline to complement p=X, ft=Y.