Add sync git hooks.

diff --git a/day1a/git_hooks/post-commit b/day1a/git_hooks/post-commit
new file mode 100755 (executable)
index 0000000..f5002cd
--- /dev/null
+++ b/day1a/git_hooks/post-commit
@@ -0,0 +1,11 @@
+#!/bin/sh
+
+set -euf
+
+remotes="$(git remote)"
+if echo "$remotes" | grep -q 'sync'; then
+    git push --all sync
+    git push --tags sync
+else
+    echo ">>>>> No sync remote defined." 1>&2
+fi