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

.config/git/README.md

@@ -0,0 +1,10 @@
+# `git`-related Configurations
+
+This folder contains
+    the [config](./config),
+    [global ignores](./ignore), and
+    the [commit message template](./commit_msg_template.txt)
+    for [`git`](https://git-scm.com/).
+
+Machine-local settings (e.g., signing keys)
+    go into an untracked `config_local`.