merge

git splice merge [--upstream <name>] [--bump] (<path>… | --all)

Splices in what was fetched, without contacting the upstream, as one ordinary commit per splice. It works like git merge --squash, done internally with a cherry-pick: on a conflict, git status shows a cherry-pick in progress (diverged history resolves one).

In the sandbox, vendor/pkg-a/'s upstream has a commit the monorepo doesn't have yet, and it's already fetched:

$ git splice merge vendor/pkg-a
ok   vendor/pkg-a: merged 703b936

The commit merge made records the fetched branch, T, as the new synced commit, U(H). Nothing was waiting to be pushed, so its folder is U(H), and it's vendor/pkg-a/'s new boundary, B:

$ git log -1 --stat --format=%s
splice: merge vendor/pkg-a from main at 703b936

 vendor/pkg-a/.splice  | 2 +-
 vendor/pkg-a/file.txt | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)

$ git config --file vendor/pkg-a/.splice splice.commit
703b9360f7ac335a1134e9b5771a90a7c09ba39a

The upstream's commits stay upstream, and in refs/splices/. The monorepo's history stays linear.

--bump also records the upstream branch in .splice when there's nothing to merge, e.g. right after a push. Use it sparingly; see bumping the sync point.