Merge (with squash) all changes from another branch as a single commit

Found it! Merge command has a --squash option

git checkout master
git merge --squash WIP

at this point everything is merged, possibly conflicted, but not committed. So I can now:

git add .
# git add -u  # might be preferable, see below
git commit -m "Merged WIP"

Another option is git merge --squash <feature branch> then finally do a git commit.

From Git merge

--squash

--no-squash

Produce the working tree and index state as if a real merge happened (except for the merge information), but do not actually make a commit or move the HEAD, nor record $GIT_DIR/MERGE_HEAD to cause the next git commit command to create a merge commit. This allows you to create a single commit on top of the current branch whose effect is the same as merging another branch (or more in case of an octopus).