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

@@ -0,0 +1,35 @@
+#!/bin/sh
+
+# List the names of all `git` aliases, one per line
+#
+# With the "--real" flag, only the "real" aliases are listed,
+# which are those with less than 7 characters in their name
+# as defined under "[alias]" in ~/.config/git/config
+#
+# Usage: git aliases [--real]
+
+set -eu
+
+
+_max_length=999
+
+
+for arg in "$@"; do
+
+    case "$arg" in
+        --real)
+            _max_length=7
+            ;;
+        *)
+            echo 'Usage: git aliases [--real]' >&2
+            exit 2
+            ;;
+    esac
+
+done
+
+
+
+git --list-cmds=alias \
+    | awk -v max="$_max_length" 'length($0) < max' \
+    | sort