This is to handle uploads that were not made with dgit, such as the uploads you made before switching to this workflow, or NMUs.
Simply prepare your git tree in the usual way, and then:
% dgit -wgf sbuild -A -c sid % dgit -wgf --overwrite push
(Do not make any tags yourself: dgit push will do that.)
You may use dgit pbuilder or dgit cowbuilder instead of dgit sbuild; see dgit(1) for the syntax of those subcommands.
The --overwrite option tells dgit that you are expecting that your git history is not a descendant of the history which dgit synthesised from the previous non-dgit uploads.
dgit will make a merge commit on your branch but without making any code changes (ie, a pseudo-merge) so that your history, which will be pushed to the dgit git server, is fast forward from the dgit archive view.
Alternatively, if this was the first ever dgit push of the package, you can avoid this merge commit by passing "--deliberately-not-fast-forward" instead of "--overwrite". This avoids introducing a new origin commit into your git history.
% dgit -wgf push
% dgit pull
Or, if you would prefer to review the changes, you can do this:
% dgit fetch % dgit diff HEAD..dgit/dgit/sid
If you do not merge the NMU into your own git history, the next push will then require --overwrite.