Indicate where git-annex will lead to pantry error

[dbaynard]

Fixes #4579.

I haven't updated the changelog; not sure where the update should go.

For tests, I could create an integration test which makes a git repo and creates a symbolic link out of the repo. It wouldn't need git-annex, but it would trigger the same error. Does that sound worthwhile?

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Note: Documentation fixes for https://docs.haskellstack.org/en/stable/ should target the "stable" branch, not master.

Please include the following checklist in your PR:

• Any changes that could be relevant to users have been recorded in the ChangeLog.md
• The documentation has been updated, if necessary.

Please also shortly describe how you tested your change. Bonus points for added tests!

[qrilka]

@dbaynard I think it would be good to have such a test. Otherwise the PR looks fine to me.

[dbaynard]

OK, it might be the weekend before I get a chance to look at this. If there's pressure to merge first, I'll submit another PR with the test.

[snoyberg]

I'm OK with this as-is, merging. Thanks!