Bug#1088327: dgit: Add `--allow-unrelated-histories' when running `git merge' in dgit pull

[Xiyue Deng]

Control: tags -1 patch

I have created a merge request on Salsa[1] with the proposed fix.

[1] https://salsa.debian.org/dgit-team/dgit/-/merge_requests/63

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Regards,
Xiyue Deng