Adjust make_traverse_dotdot_slashes.sh for environment

These are changes that do not significantly affect behavior but use
the set of tools that should be availalble in testing environments,
as well as refactorings that are useful to do not before really
making this usable as a fixture.

- Use bash shebang, enable pipefail.
- Don't require xxd.
- Don't create an extra temporary file.
- Shorten, simplify, and clarify some logic.

Author: EliahKagan

gix-worktree/tests/fixtures/make_traverse_dotdot_slashes.sh

@@ -1,28 +1,28 @@
-#!/bin/sh
-set -eu
+#!/bin/bash
+set -eu -o pipefail
 
 readonly filename='../outside'
 readonly filemode=100644
 
 emit_payload() {
-    printf 'A file outside the working tree, somehow.\n'
+    echo 'A file outside the working tree, somehow.'
 }
 
 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
+blob_hash_escaped="$(
+    emit_payload |
+    git hash-object -w --stdin |
+    sed 's/../\\x&/g'
+)"
 
 tree_hash="$(
-    printf '%s %s\0' "$filemode" "$filename" |
-    cat - blob-hash-bytes |
+    printf "%s %s\\0$blob_hash_escaped" "$filemode" "$filename" |
     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"