diff

git splice diff [--upstream <name>] [--stat] [path…]

Shows what push would send: the changes from the fetched upstream branch, T, to the rebuild, R, with paths as the upstream sees them.

$ git splice pull vendor/pkg-a >/dev/null

A commit in the monorepo changes vendor/pkg-a/:

$ echo "a local fix" >>vendor/pkg-a/file.txt && git commit -qam "pkg-a: a local fix"

$ git splice diff
===  vendor/pkg-a
diff --git a/file.txt b/file.txt
index 1b6c064..15c0cbd 100644
--- a/file.txt
+++ b/file.txt
@@ -1,2 +1,3 @@
 pkg-a: seed
 pkg-a: a second commit, after the clone
+a local fix

--stat summarizes the changes per file:

$ git splice diff --stat
===  vendor/pkg-a
 file.txt | 1 +
 1 file changed, 1 insertion(+)