Add configuration for git

- Add .config/git/config
  + Define aliases
  + Configure default behavior in various situations
  + Add user information with signing key
- Add .config/git/commit_msg_template.txt
- Add .config/git/ignore
- Integrate `git` aliases into the shell

.local/bin/git-prune-delete

@@ -0,0 +1,23 @@
+#!/bin/sh
+
+# Delete the local branches whose upstream branch was deleted on the remote
+#
+# `git branch --delete` without "--force" is used,
+# so branches holding unmerged commits are kept
+#
+# Usage: git prune-delete
+
+set -eu
+
+
+_branches=$(git prune-show)
+
+
+if [ -z "$_branches" ]; then
+    echo "No branches to prune"
+    exit 0
+fi
+
+
+# shellcheck disable=SC2086  # because branch names never contain whitespace
+git branch --delete $_branches