Merge branch 'ps/validate-prefix-in-subtree-split' into jch

"git subtree split --prefix=P <commit>" now checks the prefix P
against the tree of the (potentially quite different from the
current working tree) given commit.

* ps/validate-prefix-in-subtree-split:
  subtree: validate --prefix against commit in split

contrib/subtree/git-subtree.sh

@@ -257,6 +257,9 @@ main () {
 		test -e "$arg_prefix" &&
 			die "fatal: prefix '$arg_prefix' already exists."
 		;;
+	split)
+		# checked later against the commit, not the working tree
+		;;
 	*)
 		test -e "$arg_prefix" ||
 			die "fatal: '$arg_prefix' does not exist; use 'git subtree add'"
@@ -1001,6 +1004,12 @@ cmd_split () {
 	else
 		die "fatal: you must provide exactly one revision, and optionally a repository.  Got: '$*'"
 	fi
+
+	# Now validate prefix against the commit, not the working tree
+	if ! git ls-tree -d "$rev" -- "$dir" >/dev/null
+	then
+		die "fatal: '$dir' does not exist in commit $rev"
+	fi
 	repository=""
 	if test "$#" = 2
 	then

contrib/subtree/t/t7900-subtree.sh

@@ -368,6 +368,28 @@ test_expect_success 'split requires path given by option --prefix must exist' '
 	)
 '
 
+test_expect_success 'split works when prefix exists in commit but not in working tree' '
+	subtree_test_create_repo "$test_count" &&
+	(
+		cd "$test_count" &&
+
+		# create subtree
+		mkdir pkg &&
+		echo ok >pkg/file &&
+		git add pkg &&
+		git commit -m "add pkg" &&
+		good=$(git rev-parse HEAD) &&
+
+		# remove it from working tree in later commit
+		git rm -r pkg &&
+		git commit -m "remove pkg" &&
+
+		# must still be able to split using the old commit
+		git subtree split --prefix=pkg "$good" >out &&
+		test -s out
+	)
+'
+
 test_expect_success 'split rejects flags for add' '
 	subtree_test_create_repo "$test_count" &&
 	subtree_test_create_repo "$test_count/sub proj" &&