Ignore file headers from add sums

There are several issues and comments regarding the caching (or not) of ADDed files.

For us it was a problem that all the metadata in the files was being used (which includes not only timestamps, but also things like permissions, user names, device…).

Looking at the tests, it looks like this was never the intention, but the existing test setup did not reveal the issue.

The first commit in the PR is used to make the test fail (just add a non-constant timestamp to the test data files).
The second commit is the actual fix.

Fixes #3699