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/ignore

@@ -0,0 +1,6 @@
+# Generic temporary files
+*.backup
+*.bak
+*.orig
+*.temp
+*.tmp