We now have two `cmd_show` in the repo. I think this one should become
`cmd_file_show`, but this should be done uniformly over all the commands
for consistency.
I did *not* keep `print` as an alias (I couldn't find a compelling
reason to do it), but let me know if anyone feels like keeping it.
Some backends, like the one we have at Google, can restrict access to
certain files. For such files, if they return a regular
`BackendError::ReadObject`, then that will terminate iteration in many
cases (e.g. when diffing or listing files). This patch adds a new
error variant for them to return instead, plus handling of such errors
in diff output and in the working copy.
In order to test the feature, I added a new commit backend that
returns the new `ReadAccessDenied` error when the caller tries to read
certain objects.
