Omit .git directory in guest.pull() during prepare.distgit

[lukaszachy]

Found by 'pcs': rpmbuild uses git to apply patches, and newer git run maintenance automatically. tmt was running into race condition where some git objects were deleted after rsync from guest.pull was already aware of them, which lead to broken prepare with
sync warning: some files vanished before they could be transferred (code 24)

We don't need the .git directory anyway so we will save few bits in the
download as well.

• https://git-scm.com/docs/git-maintenance#Documentation/git-maintenance.txt-maintenanceauto

Fixes: #4893

Pull Request Checklist

• implement the feature

[gemini-code-assist]

Code Review

This pull request modifies tmt/steps/prepare/distgit.py to exclude the .git directory during source retrieval to avoid rsync errors. Feedback suggests appending to the exclude list rather than overwriting it to preserve default patterns.

[thrix]

Seems some copr outage

[thrix]

/packit build

[happz]

Seems some copr outage

Don't forget to mention it on our "Annoying issues" canvas, please.

[LecrisUT]

I am not sure about this one. The original issue is more of a misunderstanding of the scope of %autosteup -S git. We have other git-like files that are synced like the test tree, and if we are going to do such a change, I would prefer this to be consistent. I don't think we want to remove the git from the test tree because the test might be using it, but by the same logic, if a project really needed the git repo in the %prep how do we know it is not needed for the tests also.