While the change view handles new params, it may set the patch range to
an incoherent state while the change itself is being updated. If the
file list contains expanded diffs, they can try to reload in this
incoherent state and potentially encounter network errors when loading
nonexistent diffs.
With this change, the file list's `collapseAllDiffs` method is made
public, and is invoked by the change view's `_paramsChanged` method
before internal state reconfiguration.
Bug: Issue 6969
Change-Id: I8a400e6eccc5002b206c24153071c142f2f2fc35
f4f6101c62