Even if it didn't handle commits across subtrees, I would still find this useful. I would be ok if this (or an addon) tool detected commits that that spanned s

Even if it didn't handle commits across subtrees, I would still find this useful. I would be ok if this (or an addon) tool detected commits that that spanned subprojects and invited me to split that commit. IMHO, the correct workflow should be: check in modifications to the library subtree and to the library's client in different commits.