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-checkout-dev

@@ -0,0 +1,8 @@
+#!/bin/sh
+
+# Check out the "develop" branch, whatever it is called
+
+set -eu
+
+
+exec git checkout-first-existing-branch dev develop development