tests: check hardlinking in case of local download from Git working copy
The download recipe relies on buildout's download, which uses hardlinking for performance reasons in 2 cases: downloading from cache or from local file. This means 2 things: - the user shall not modify downloaded files without first making sure that st_nlink == 1 - the source file shall not be modified in-place, which is reasable to assume for both cache and local file In-place modification of files is more and more rare because it has many drawbacks and it tends to be limited to things like logs and databases. This commit adds a test to check that Git does not do that when managing working copies: and actually, this may be our only use case of local download. This commit does not mean that the current way of hardlinking is fully fine. There remains at least the issue that file permissions may be changed at the end of the recipe, either with `shared=true` or with `mode` option.
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