git: Rename zsh-autoload.sh -> external-script.sh

With the last commit 9c1e3f4679 (git:zsh-autoload: Use relative
scripts/ folder, 2025-09-12), `zsh-autoload.sh` could execute any type
of external script. Rename it to a more generic name.

.config/git/external-script.sh

@@ -0,0 +1,28 @@
+#!/bin/sh
+# SPDX-License-Identifier: MIT
+# Copyright (c) 2025 Julian Prein
+#
+# Meant to be used by git aliases to easily launch an external script from the
+# ./scripts/ collection through its basename and in the correct directory (i.e.
+# GIT_PREFIX).
+
+if [ $# -eq 0 ]; then
+	printf >&2 "Usage: %s <function>\n" "$(basename "$0")"
+	exit 1
+fi
+name="$1"
+shift
+
+BASE="$(dirname "$(realpath "$0")")/scripts"
+
+# In git aliases, shell commands are executed from the top-level directory of
+# the repo. GIT_PREFIX contains the original directory relative to the
+# top-level.
+[ -z "$GIT_PREFIX" ] || cd "$GIT_PREFIX" || exit
+
+# no need for error handling, the message from sh is descriptive enough
+if [ "${name#git-}" != "$name" ] || [ -e "$BASE/$name" ]; then
+	exec "$BASE/$name" "$@"
+else
+	exec "$BASE/git-$name" "$@"
+fi