[mono] Simplify file comparisons

[lewing]

It's late on a Friday so it is likely I'm missing something, but the hash comparison doesn't appear to be useful at all, that said I will assume it is useful and preserve the original behavior for now.

Use two rolling buffers to do file equality comparison since Read can return less than a full buffer, only advance a buffer once it is consumed, and compare the overlapping region.

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src/tasks/Common/Utils.cs:217

• Ensure that the new behavior introduced by the CompareFiles method is covered by tests. This includes scenarios where files are identical and where they differ.

private static bool CompareFiles(string filePath1, string filePath2)

[lewing]

This condition appears to be incorrect given that even when useHash is true and they the hashes differ it will fall back to checking the text. It can either be dropped completely to compare the text or it can skip the text check if use hash is true.

[akoeplinger]

yeah I think the intention was to use the (fast) hash comparison and only fallback to comparing the text if the hash differs.

so this could likely be

bool areSame = File.Exists(dst) && (useHash ? ComputeHash(src) != ComputeHash(dst) : File.ReadAllText(src) != File.ReadAllText(dst);

but I wonder too why we don't just always compare the hash.