id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
770937642,MDU6SXNzdWU3NzA5Mzc2NDI=,4708,Potentially spurious warning in rechunk,1312546,closed,0,,,0,2020-12-18T14:37:32Z,2020-12-24T11:32:43Z,2020-12-24T11:32:43Z,MEMBER,,,,"**What happened**:

When reading an zarr dataset where the last chunk is smaller than the chunk size, users see a `UserWarning` that this may be inefficient, since the chunking differs from the chunking on disk. In general that's a good warning, but it shouldn't appear when the only difference between the on-disk chunking and the Dataset chunking is the last chunk.

**What you expected to happen**:

No warning.

**Minimal Complete Verifiable Example**:

```python
# Create and write the data
import numpy as np
import pandas as pd
import xarray as xr

np.random.seed(0)
temperature = 15 + 8 * np.random.randn(2, 2, 3)
precipitation = 10 * np.random.rand(2, 2, 3)
lon = [[-99.83, -99.32], [-99.79, -99.23]]
lat = [[42.25, 42.21], [42.63, 42.59]]
time = pd.date_range(""2014-09-06"", periods=3)
reference_time = pd.Timestamp(""2014-09-05"")
ds = xr.Dataset(
    data_vars=dict(
        temperature=([""x"", ""y"", ""time""], temperature),
        precipitation=([""x"", ""y"", ""time""], precipitation),
    ),
    coords=dict(
        lon=([""x"", ""y""], lon),
        lat=([""x"", ""y""], lat),
        time=time,
        reference_time=reference_time,
    ),
    attrs=dict(description=""Weather related data.""),
)
ds2 = ds.chunk(chunks=dict(time=(2, 1)))
ds2['temperature'].chunks

ds2.to_zarr(""/tmp/test.zarr"", mode=""w"")
```

Reading it produces a warning

```python
xr.open_zarr(""/tmp/test.zarr"")
/mnt/c/Users/taugspurger/src/xarray/xarray/core/dataset.py:408: UserWarning: Specified Dask chunks (2, 1) would separate on disks chunk shape 2 for dimension time. This could degrade performance. Consider rechunking after loading instead.
  _check_chunks_compatibility(var, output_chunks, preferred_chunks)
```

**Anything else we need to know?**:

The check around https://github.com/pydata/xarray/blob/91318d2ee63149669404489be9198f230d877642/xarray/core/dataset.py#L371-L378 should probably ignore the very last chunk, since Zarr allows it to be different?

**Environment**:

<details><summary>Output of <tt>xr.show_versions()</tt></summary>

INSTALLED VERSIONS
------------------
commit: None
python: 3.8.6 | packaged by conda-forge | (default, Oct  7 2020, 19:08:05) 
[GCC 7.5.0]
python-bits: 64
OS: Linux
OS-release: 4.19.128-microsoft-standard
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: C.UTF-8
LOCALE: en_US.UTF-8
libhdf5: None
libnetcdf: None

xarray: 0.16.3.dev21+g96e1aea0
pandas: 1.1.4
numpy: 1.19.4
scipy: 1.5.4
netCDF4: None
pydap: None
h5netcdf: None
h5py: None
Nio: None
zarr: 2.6.2.dev9+dirty
cftime: 1.3.0
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: None
dask: 2.30.0
distributed: None
matplotlib: None
cartopy: None
seaborn: None
numbagg: None
pint: None
setuptools: 49.6.0.post20201009
pip: 20.2.4
conda: None
pytest: 5.4.3
IPython: 7.19.0
sphinx: None


</details>
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