Start on demo script making repo with ../… filename

The repo this script makes contains a filename with a slash character
in it that, if not rejected, will create a file above the working tree.

This is a modification of make_traverse_dotgit_slashes.sh. Both
require some further revision, and since most of their content is
duplicated, it may be worthwhile to combine them to avoid that.

Author: EliahKagan

gix-worktree/tests/fixtures/make_traverse_dotdot_slashes.sh

@@ -0,0 +1,29 @@
+#!/bin/sh
+set -eu
+
+readonly filename='../outside'
+readonly filemode=100644
+
+emit_payload() {
+    printf 'A file outside the working tree, somehow.\n'
+}
+
+repo="$1"
+git init -- "$repo"
+cd -- "$repo"
+
+blob_hash="$(emit_payload | git hash-object -w --stdin)"
+printf '%s' "$blob_hash" | xxd -r -p >blob-hash-bytes
+
+tree_hash="$(
+    printf '%s %s\0' "$filemode" "$filename" |
+    cat - blob-hash-bytes |
+    git hash-object -t tree -w --stdin --literally
+)"
+
+rm blob-hash-bytes
+
+commit_hash="$(git commit-tree -m 'Initial commit' "$tree_hash")"
+branch="$(git symbolic-ref --short HEAD)"
+git branch -f -- "$branch" "$commit_hash"
+git show