Implement a new caching system to make repeated runs much much faster.

hash.h

@@ -0,0 +1,37 @@
+#pragma once
+
+#include <stdint.h>
+#include <stdbool.h>
+#include <git2/repository.h>
+
+#ifdef GIT
+typedef struct {
+  uint64_t hash;
+  bool changes;
+} repohash;
+
+#define XD_HASH_PATH "/XDhash"
+/**
+ * @brief generate a hash from the repository state
+ *
+ * @param repo the git repository
+ * @return the hash
+ */
+uint64_t generate_hash(git_repository *repo);
+
+/**
+ * @brief read the hash from the git repo
+ *
+ * @param repo the git repository
+ * @return the hash
+ */
+repohash *read_hash(git_repository *repo);
+
+/**
+ * @brief write a new hash to the repository
+ *
+ * @param repo the repository
+ * @param hash the hash to write
+ */
+int write_hash(git_repository *repo, repohash hash);
+#endif