Standardize REST endpoints for getting changes

The /detail and /review endpoints are practically identical to the
standard change endpoint except for the default set of options. Make
this equivalence explicit in the code by delegating to a GetChange
instance from the other handlers.

This change naturally gets caching for the other endpoints as well.

Allow specifying options on /change/X; prior to this, users could add
options to the default set from /detail but could not remove options.

Change-Id: I03bb96141e79ae00359f8d5ec51413d1b27204dc
5 files changed