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-checkout-first-existing-branch

@@ -0,0 +1,28 @@
+#!/bin/sh
+
+# Check out the first of the given branches that exists,
+# looking at both local branches and the ones on `origin`
+#
+# Usage: git checkout-first-existing-branch <branch> [<branch> ...]
+
+set -eu
+
+
+if [ $# -eq 0 ]; then
+    echo "Usage: git checkout-first-existing-branch <branch> [<branch> ...]" >&2
+    exit 2
+fi
+
+
+for _branch in "$@"; do
+
+    if git show-ref --verify --quiet "refs/heads/$_branch" \
+    || git show-ref --verify --quiet "refs/remotes/origin/$_branch"; then
+        exec git checkout "$_branch"
+    fi
+
+done
+
+
+echo "None of these branches exists: $*" >&2
+exit 1