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-aliases-n-sub-commands

@@ -0,0 +1,15 @@
+#!/bin/sh
+
+# List the names of all `git` aliases and custom sub-commands, one per line
+#
+# Usage: git aliases-n-sub-commands
+
+set -eu
+
+
+{
+
+    git --list-cmds=alias
+    git --list-cmds=others
+
+} | sort -u