Ian Jackson [off-list ref] writes:

+# Usage: reject_if_v2_config REV
+#
+# Bails if we find .git-subtree/config.  This file is used by the RIIR
+# git-subtree, which can read data from this script, but which generates
+# data that this script cannot cope with.  So if we find that the user's
+# project has already been processed with the new tool, we stop, to
+# avoid generating broken output.
+reject_if_v2_config () {
+	local config=.git-subtree/config
+	if git rev-parse --verify -q "$rev:$config"; then
+		die "fatal: tree contains $config: has been processed with new standalone (Rust) git-subtree; use that tool instead of this one.  See https://codeberg.org/diziet/git-subtree https://crates.io/crates/git-subtree"
+	fi
+}

[warning: I have no idea what is going on in the code we see here,
as I do not use subtree script at all]

The above helper may work for one caller that passes "$rev" but not
for the other caller that passes "HEAD", no?


	if git rev-parse --verify -q "$1:$config"
	then
		die "fatal: tree contains $config: has been processed with new standalone (Rust) git-subtree; use that tool instead of this one.  See https://codeberg.org/diziet/git-subtree https://crates.io/crates/git-subtree"
	fi

Overly long output does not look very easy to read, but I kept it
the same as the original.

If (global) $rev is not set here, we'd check :.git-subtree/config in
the index in order to detect the v2's configuration.  It seems to me
that this code however wants to inspect HEAD's tree.

This would happen to work, as the global "$rev" visible here is the
same one as what the new helper function sees and uses.

The same comment as the one for cmd_add's usage.

 	if test -n "$arg_addmerge_squash"