merge in vendor branch after "gbp import-orig --no-merge"

[Fab Stz <fabstz-it@yahoo.fr>]

Hello,

On my repo (on salsa) I usually run

gbp import-orig --uscan --pristine

I would like to be able to update both pristine and upstream branch when there 
is a new upstream version but not the debian/latest branch. Reason is that 
<vendor>/latest branches an debian/latest branches may have a different update 
pace.

So I found we could run:

gbp import-orig --uscan --pristine --no-merge

Now my question is:

How to update the vendor branches individually with the latest upstream branch 
(or a given upstream version) and have the same result as a call without '--
no-merge' (which would do a --merge-mode=replace" in my case?

There is "git merge upstream/<version>" but the result is not equally the 
same, at least in the log messages, as when runing gbp import-orig with merge 
enabled (merge-mode is auto, which in my case becomes 'replace').

(Please, leave me in copy of your answer).

Thanks
Fab