Add test: pinned git deps are not changed

[teodorlu]

Please answer the following questions and leave the below in as part of your PR.

• [ ] This PR corresponds to an issue with a clear problem statement.

  • I believe https://github.com/babashka/neil/issues/230 is actually not a bug, but we might as well cover the case in question against future regressions.

• [ ] I have updated the CHANGELOG.md file with a description of the addressed issue.

  • I don't believe a CHANGELOG entry is warranted.

• [x] I have considered whether I should add more tests covering the code I've changed.

  • This PR contains a test.

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I'm unable to reproduce https://github.com/babashka/neil/issues/230 locally.

• This test is green on my computer
• neil-dev dep upgrade works as I expect (run from latest master)
• neil dep upgrade works as I expect (neil 0.3.67)

But I don't think a test covering the case would hurt.

[borkdude]

Thanks !