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-point-branch-at

@@ -0,0 +1,21 @@
+#!/bin/sh
+
+# Move a branch to a commit, whether or not it is checked out
+#
+# Usage: git point-branch-at [<branch>] <commit>
+
+set -eu
+
+
+case $# in
+    1) _branch=$(git rev-parse --abbrev-ref HEAD); _commit=$1 ;;
+    2) _branch=$1; _commit=$2 ;;
+    *) echo "Usage: git point-branch-at [<branch>] <commit>" >&2; exit 2 ;;
+esac
+
+
+if [ "$_branch" = "$(git rev-parse --abbrev-ref HEAD)" ]; then
+    exec git reset --keep "$_commit"
+else
+    exec git branch --force "$_branch" "$_commit"
+fi