Outline what the first version of the upgrade script should do

upgrade.sh

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+#!/bin/bash
+# shellcheck disable=SC2002
+
+set -eu
+set -o pipefail
+IFS=$'\n\t'
+
+# print useful message on failure
+trap 's=$?; echo >&2 "$0: Error on line "$LINENO": $BASH_COMMAND"; exit $s' ERR
+
+# shellcheck disable=SC2034
+DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+
+cd "$DIR"
+
+# - Check if the file .diachron-version exists; save its value in a variable
+#   named old_diachron_version
+
+# - Check if the repository is dirty; if there are any files that git knows
+#   about that have been changed but not committed, abort with a message
+
+# - Get the current commit and store it in a variable
+
+# - Perform a two checkouts of
+#   https://gitea.philologue.net/philologue/diachron, each in its own
+#   temporary directory.  We'll call one "old" and one "new"
+
+# - In old, check out $old_diachron_version; in our working application
+#   directory, delete all of the files in file-list using git rm
+
+# - In new, check out whatever was passed as argv[1]
+
+# - Copy all of the files in file-list to the working application and stage
+#   them with git add
+
+# - Commit
+
+# - Should we run sync.sh or should we advise the user to run sync.sh?