Combine "slashes" scripts and make it a fixture

Keeping the changes to make_traverse_dotdot_slashes.sh, this folds
the make_traverse_dotgit_slsahes.sh logic into it, extracting the
twice-used parts (which are most of the script) into a function
that both call. Other changes:

- No longer use command-line arguments. There are two repositories
  that are currently useful to make in this way, and this calls the
  function for each of them.

- Change the style to mostly match that of other fixture scripts,
  including decreasing the indent from 4 to 2 and using the
  function keyword when defining functions.

- Shorten variable names in cases where doing so is unambiguous
  (but not otherwise).

- Eliminate the emit_payload function, since the new make_repo
  function now receives the content on standard input, which can
  be provided by whatever means is convenient (the current calls
  use a here string for the one-line file and a heredoc otherwise).

Author: EliahKagan

gix-worktree/tests/fixtures/make_traverse_dotdot_slashes.sh

@@ -0,0 +1,35 @@
+#!/bin/bash
+set -eu -o pipefail
+
+# Makes a repo carrying a literally named file, which may even contain "/".
+# File content is from stdin. Arguments are repo name, file name, and file mode.
+function make_repo() (
+  local repo="$1" file="$2" mode="$3"
+  local blob_hash_escaped tree_hash commit_hash branch
+
+  git init -- "$repo"
+  cd -- "$repo" # Temporary, as the function body is a ( ) subshell.
+
+  blob_hash_escaped="$(git hash-object -w --stdin | sed 's/../\\x&/g')"
+
+  tree_hash="$(
+    printf "%s %s\\0$blob_hash_escaped" "$mode" "$file" |
+    git hash-object -t tree -w --stdin --literally
+  )"
+
+  commit_hash="$(git commit-tree -m 'Initial commit' "$tree_hash")"
+
+  branch="$(git symbolic-ref --short HEAD)"
+  git branch -f -- "$branch" "$commit_hash"
+  test -z "${DEBUG_FIXTURE-}" || git show # TODO: How should verbosity be controlled?
+)
+
+make_repo traverse_dotdot_slashes ../outside 100644 \
+  <<<'A file outside the working tree, somehow.'
+
+# TODO: Should the payload be simplified to a single side effect for tests to check?
+make_repo traverse_dotgit_slashes .git/hooks/pre-commit 100755 <<'EOF'
+#!/bin/sh
+printf 'Vulnerable!\n'
+date >vulnerable
+EOF