fix(dev-infra): merge script autosquash should be interactive (#37138)
fix(dev-infra): merge script autosquash should be interactive (#37138)

The components repo does not use the autosquash merge strategy, so
recent changes to that seem to broke the autosquash strategy.

Since we don't run the rebase in interactive mode, the `--autosquash`
flag has no effect. This is by design in Git. We can make it work by
setting the git sequence editor to `true` so that the rebase seems
like an interactive one to Git, while it isn't one for the user.

This matches conceptually with the merge script currently used in
framework. The only difference is that we allow a real interactive
rebase if the `commit message fixup` label is applied. This allows
commit message modifications (and others) if needed.

PR Close #37138

angular/dev-infra,angular/dev-infra,angular/dev-infra,angular/dev-infra