Always reset the git repostiory after adding gitattributes

Commit 477e59b5 "Reset the git repostiory after adding gitattributes" ensured on the pull-mirror job that the checked out contents match the current gitattributes configuration.

This extends such guarantee to all jobs, avoiding situations like:

1. the upstream .gitattributes enables the ident filter on a file
2. on checkout by the CI, $Id$ placeholders are replaced
3. we defuse the .gitattributes, but it is too late since the working copy is already dirty
4. jobs like pull-updates fail due to the unexpected changes