git: Fix working directory of shell aliases

In git aliases, shell commands are executed from the top-level directory
of the repo. Because of this, something like `git glog -- file` didn't
work in subdirectories.

Fix this by creating a wrapper script that first changes into
$GIT_PREFIX before executing the script. Since (currently) the only
use-case is to launch the functions in `autoload/git/`, most of the path
could move into the script, making the git config a lot cleaner.

.config/git/zsh-autoload.sh

@@ -0,0 +1,25 @@
+#!/bin/sh
+# SPDX-License-Identifier: MIT
+# Copyright (c) 2025 Julian Prein
+#
+# Meant to be used in git aliases to launch an autoloadable zsh function in the
+# correct directory.
+
+if [ $# -eq 0 ]; then
+	printf >&2 "Usage: $(basename "$0") <function>\n"
+	exit 1
+fi
+
+BASE="${XDG_CONFIG_HOME:-$HOME/.config}/zsh/autoload/git"
+
+# In git aliases, shell commands are executed from the top-level directory of
+# the repo. GIT_PREFIX contains the original directory relative to the
+# top-level.
+[ -z "$GIT_PREFIX" ] || cd "$GIT_PREFIX"
+
+# no need for error handling, the message from sh is descriptive enough
+if [ "${1#git-}" != "$1" ] || [ -e "$BASE/$1" ]; then
+	exec "$BASE/$@"
+else
+	exec "$BASE/git-$@"
+fi