timeline: Avoid creating extra transition when rippling clips

timeline: Avoid creating extra transition when rippling clips

In some cases when rippling clip we could get the algo lost because
a transition existed between two clips (for example at the end of c1
and at the begining of c2) but while rippling it would have required
a transition at the end of c2 and beginning of c1, and we were properly
not destroying the old one (as the two clips were in the moving context)
but we were still creating the other transition in the end...

Reviewed-by: Alex Băluț <alexandru.balut@gmail.com>
Differential Revision: https://phabricator.freedesktop.org/D1362