With groupChanges, existing change file for branch should be updated if it exists

With the groupChanges option in a monorepo, beachball will currently create a new file each time you run yarn change (with additional changed packages) even if a multi-package change file already existed for that branch.

Repro:

1. Set groupChanges: true in beachball config
2. Change some packages and commit
3. Run beachball to create a change file
4. Change some more packages and commit
5. Run beachball again

Actual: a new change file is created

Expected: new changes should be added to the existing change file for that branch

Important edge case: must verify that the existing file is for multiple packages (has changes key) before adding to it. Otherwise fall back to creating a new file. (This is important for repos which recently enabled groupChanges and may still have open PR branches with single-package change files.)

cc @jcreamer898 -- this is more of an enhancement than a bug, but if you have time and don't mind working on it that would be great (not sure if/when I'd get to it)

Start with the groupChanges handling used by yarn change and trace how existing change files are selected. Reproduce the two-run scenario, then verify that an existing file is updated only when it has a changes key; otherwise a new file should be created. Done means both the multi-package and single-package edge cases behave as expected.