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-show

@@ -0,0 +1,15 @@
+#!/bin/sh
+
+# Show the local branches whose upstream branch was deleted on the remote
+#
+# Usage: git prune-show
+
+set -eu
+
+
+git fetch --prune
+
+git branch \
+    --format='%(refname:short) %(upstream:track)' \
+    --list \
+    | awk '$2 == "[gone]" { print $1 }'