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-clean-all

@@ -0,0 +1,54 @@
+#!/bin/sh
+
+# Reset the working directory to `HEAD` and remove everything not tracked,
+# except the files listed in $KEEP in ~/.local/bin/git-clean-ignored
+#
+# This is irreversible, so the changes are shown first and confirmed,
+# unless "--force" is passed
+#
+# Usage: git clean-all [--force]
+
+set -eu
+
+
+# `git` runs scripts on the `$PATH` in the current directory,
+# whereas it runs "!" aliases at the top level of the repository
+cd "$(git rev-parse --show-toplevel)"
+
+
+case "${1:-}" in
+    --force) _forced=1 ;;
+         "") _forced=0 ;;
+          *) echo "Usage: git clean-all [--force]" >&2; exit 2 ;;
+esac
+
+
+if [ "$_forced" -ne 1 ]; then
+
+    echo "Tracked files to be reset to HEAD:"
+    git diff --stat HEAD || true
+    echo ""
+
+    echo "Untracked and ignored files to be removed:"
+    # Keep the "-e" flags in sync with the other two copies:
+    #  - the "-e" flags of `clean-untracked` in ~/.config/git/config
+    #  - `$KEEP` in ~/.local/bin/git-clean-ignored
+    git clean -x -d -n -e ".python-version" -e ".env"
+    echo ""
+
+    printf "This CANNOT be undone. Continue? [y/N] "
+    read -r _answer || _answer=""
+
+    case "$_answer" in
+        [yY] | [yY][eE][sS]) ;;
+        *) echo "Aborted" >&2; exit 1 ;;
+    esac
+
+fi
+
+
+git reset --hard
+
+git clean-ignored
+
+git clean-untracked