fix(v-model): update model correctly when input event is bound #6690

[chriscasola]

Use the "input" event instead of "change" when listening for
changes to the selection state since the "input" event will
always fire before the "change" event.

This avoids an issue where a user binds to the "input" event
with "v-bind" and causes the component to update and set its
model value back to the value of the select before it has
received the new selection value.

Fixes #6690

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

There is a detailed description of what causes this bug in #6690 (comment)

[yyx990803]

Thanks again for the PR - however, I don't think switching events would solve the root problem, which is that microtasks (which Vue uses for async deferring) can fire in between these supposedly sequential events.

I managed to find a nextTick implementation that fires after all sequential events in 6e41679.