[PATCH v2] subtree: validate --prefix against commit in split

[Pushkar Singh <pushkarkumarsingh1970@gmail.com>]

git subtree split currently validates --prefix against the working tree.
This breaks when splitting an older commit or when the working tree does
not contain the subtree, even though the commit does.

For example:

  git subtree split --prefix=pkg <commit>

fails if pkg was removed later, even though it exists in <commit>.

Fix this by validating the prefix against the specified commit using
git ls-tree instead of the working tree.

Signed-off-by: Pushkar Singh <pushkarkumarsingh1970@gmail.com>
---
 contrib/subtree/git-subtree.sh | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/contrib/subtree/git-subtree.sh b/contrib/subtree/git-subtree.sh
index 17106d1a72..a5822b66d5 100755
--- a/contrib/subtree/git-subtree.sh
+++ b/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'"
@@ -966,6 +969,12 @@ 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
-- 
2.43.0