Use GIT_CREDENTIAL_CALLBACK=0 to disable use of credential callback

[ianthomas23]

This adds the ability to disable the use of the libgit2 credential callback which requests a username and password from the user, using an environment variable GIT_CREDENTIAL_CALLBACK=0. Possible use cases for this are:

• Limiting access to remote repositories that do not need authentication.
• Downstream projects which implement their own authentication, perhaps via a CORS proxy or some other interception of the remote https request.

I considered a number of other possible environment variables:

• GIT_DISABLE_CREDENTIALS=1
• GIT_DISABLE_CREDENTIAL_CALLBACK=1
• GIT_NO_CREDENTIAL_CALLBACK=1

and in the end I have opted for what I think is the least bad option. I could easily be persuaded to use a different one.

If you disable the callback then a request that requires authentication will fail with the following error message:

error: remote authentication required but no callback set

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.18%. Comparing base (760ca88) to head (294cbb9).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #172      +/-   ##
==========================================
+ Coverage   88.16%   88.18%   +0.02%     
==========================================
  Files          63       63              
  Lines        2991     2997       +6     
  Branches      373      376       +3     
==========================================
+ Hits         2637     2643       +6     
  Misses        354      354              

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[ianthomas23]

Wasm tests all pass https://github.com/QuantStack/git2cpp/actions/runs/28231520400/job/83636164940 so this is good for review now.