With a proxy, snap info endpoint fails if "revision" not in requested fields

[Jonathan Hartley]

Given a store proxy.

When a call is made to /v2/snap[s/info/{name}?fields=snap-id
(ie. any set of fields that does not include 'revision')

Then the a 200 response is expected with the specified fields,
but instead we get an error response.

I'm unhelpfully not specifying the error because I haven't reproduced this against a running server, only seen it happen in the tests:

eg. in devicegw source, modify the assignment of 'fields' at the start of OverrideWithLocalSnaprevsTests.test_specific_fields, to remove 'revision'. This results in:

```
ERROR: test_specific_fields (snapdevicegw.tests.test_webapi_info.OverrideWithLocalSnaprevsTests)
snapdevicegw.tests.test_webapi_info.OverrideWithLocalSnaprevsTests.test_specific_fields
----------------------------------------------------------------------
testtools.testresult.real._StringException: Traceback (most recent call last):
  File "/home/jhartley/src/snapdevicegw/snapdevicegw/tests/test_webapi_info.py", line 581, in test_specific_fields
    response = self.do_request(snap_name, query=query)
  File "/home/jhartley/src/snapdevicegw/snapdevicegw/tests/test_webapi_info.py", line 416, in do_request
    response = self.fixture.client.get(urlpath, headers=headers)
... [snip] ...
  File "/home/jhartley/src/snapdevicegw/snapdevicegw/webapi_info.py", line 159, in get_snap_info
    results = _override_with_local_snaprevs(snap_name, upstream_data)
  File "/home/jhartley/src/snapdevicegw/snapdevicegw/webapi_info.py", line 241, in _override_with_local_snaprevs
    snap_name, snap_id, snap_ident_data, channel_map)
  File "/home/jhartley/src/snapdevicegw/snapdevicegw/proxy.py", line 368, in override_snap_info
    upstream_channel_map)
  File "/home/jhartley/src/snapdevicegw/snapdevicegw/proxy.py", line 325, in _transform_snap_info_external_to_internal
    'revision': up_item['revision'],
KeyError: 'revision'
```

The source there indexes by 'revision', instead of using .get() as is done for other fields on adjacent lines, but a quick skim of the source there looks nontrivial to fix, so I'm filing a bug for now.

I'm a newhire, please shout at me if this bug report is lacking in some way. Thanks.

[Przemysław Suliga (suligap)]

This is fixed. It was tracked in a duplicate of this bug at https://bugs.launchpad.net/snapstore-snap/+bug/1942892