I want to check the diff between current master branch and an old git tag (lets say, tag1).
After checking the differences, I want to discard the changes and merge tag1 into my master branch because tag1 is currently running in production and I want my master to be updated with it.
How can I do these tasks?
You could use git revert, assuming tag1 is an older commit on the master branch:
git revert tag1..master
Or you can use git reset (with the --soft option):
git switch master
git branch tmp master
git reset --hard tag1
git reset --soft tmp
git add .
git commit -m "commit tag1 content to master"