Squashing branch (let it be feature) commits into a single commit and rebasing them on top of target branch (let it be master) is one of the most common operations. If there is a single commit, it's trivial, if two - trivial as well (e.g. amend to parent commit first). But in many cases the feature branch can be pretty long and contain merges from target branch as well - standard situation for long PRs and active target branch. So at some point author may want to squash all changes.

TLDR:
Introduce a standard concise command:

git checkout feature
git fetch origen master
git rebase --squash origen/master

Why this could be convenient: in case of possible merge commits they will be solved only once, not a nightmare rebasing all commits.

With current available options I do it this way

git checkout feature
git fetch origen master
git merge origen/master --no-edit && git reset --soft origen/master
git commit -m "FEA-123 - new feature"

It's pretty straightforward, but this flow has pitfalls when you try to explain it to someone else. The most critical case is possible merge conflict:

# fails because of the conflict
git merge origen/master --no-edit && git reset --soft origen/master
# resolve conflict, commit
git reset --soft origen/master
# danger here: it could be possible that author did not notice that origen/master was updated (e.g.
# it was implicit background action by GIT UI client)
git commit -m "FEA-123 - new feature and revert some commits from origen/master"

To avoid the case with updated master I always execute this pair:

git merge origen/master --no-edit && git reset --soft origen/master

There is another way to do it

git checkout feature
git fetch origen master
git branch old-feature
git reset --hard origen/master
git merge old-feature --squash --no-edit
# resolve possible conflict, commit

it's already better, but still not perfect - we introduce temporary branch (or store commit hash somehow).

One more way

git checkout feature
git fetch origen master
git merge-base HEAD origen/master
git reset --soft 00_hash_from_merge_base_00 && git commit -m "FEA-123 - new feature"
git rebase origen/master

Nice for geeks.

I understand, that these scenarios can be automated via shell scripts and even custom git commands and even shared via repo.
But I still believe it could be a nice feature.

I'm very sorry, if it's a duplicate, or the feature is already here, but I could not find and proof for such idea.