commit-from: Also copy/convert existing static deltas

When copying a commit, also bring forward any static deltas.
This is particularly interesting for flathub where we can
then generate static deltas on the build machines and then import
and sign it on the repo machine.

Closes: #1409
Approved by: alexlarsson
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f