Bugfix - Write-Enabled checkout with commit argument raises exception

[rlizzo]

Motivation and Context

Why is this change required? What problem does it solve?:

explicitly raise error if commit arg is set while checking out a repository with write=True

If it fixes an open issue, please link to the issue here:

• closes [BUG REPORT] repo.checkout accepts commit argument when write=True #164

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Documentation update
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Is this PR ready for review, or a work in progress?

• Ready for review
• Work in progress

How Has This Been Tested?

Put an x in the boxes that apply:

• Current tests cover modifications made
• New tests have been added to the test suite
• Modifications were made to existing tests to support these changes
• Tests may be needed, but they are not included when the PR was proposed
• I don't know. Help!

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have signed (or will sign when prompted) the tensorwork CLA.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #166 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #166      +/-   ##
==========================================
- Coverage   95.16%   95.14%   -0.02%     
==========================================
  Files          65       65              
  Lines       11821    11831      +10     
  Branches     1023     1024       +1     
==========================================
+ Hits        11249    11256       +7     
  Misses        384      384              
- Partials      188      191       +3

Impacted Files	Coverage Δ
src/hangar/repository.py	97.64% <100%> (+0.04%)	⬆️
tests/test_checkout.py	99.81% <100%> (ø)	⬆️
src/hangar/metadata.py	92.91% <0%> (-2.36%)	⬇️
src/hangar/remote/server.py	77.29% <0%> (-1.59%)	⬇️
src/hangar/remote/client.py	80.29% <0%> (-0.87%)	⬇️
tests/test_remotes.py	98.52% <0%> (+2.03%)	⬆️