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-ff-all

@@ -0,0 +1,32 @@
+#!/bin/sh
+
+# Fast-forward all local branches that track a remote branch
+#
+# Branches with local-only commits are skipped
+#
+# Usage: git ff-all
+
+set -eu
+
+
+git fetch --all --prune
+
+
+current="$(git branch --show-current)"
+
+
+git for-each-ref --format='%(refname:short) %(upstream:short)' refs/heads/ |
+while read -r branch upstream; do
+
+    [ -n "$upstream" ] || continue
+
+    if [ "$branch" = "$current" ]; then
+        git merge --ff-only "$upstream" 2>/dev/null \
+            || echo "Skipped $branch (not a fast forward)"
+        continue
+    fi
+
+    git fetch . "$upstream:$branch" 2>/dev/null \
+        || echo "Skipped $branch (not a fast forward)"
+
+done