Adding object table (#1)

* code for maintaining the object table

* Makefile fix

* Clone git submodules.

* directory -> object_table

* Fix Makefile and remove unnecessary files.

* Fix formatting.

* make code more generic

common.c

@@ -0,0 +1,15 @@
+#include "common.h"
+
+char *sha1_to_hex(const unsigned char *sha1, char *buffer) {
+  static const char hex[] = "0123456789abcdef";
+  char *buf = buffer;
+
+  for (int i = 0; i < UNIQUE_ID_SIZE; i++) {
+    unsigned int val = *sha1++;
+    *buf++ = hex[val >> 4];
+    *buf++ = hex[val & 0xf];
+  }
+  *buf = '\0';
+
+  return buffer;
+}