Restructure db schema

The "commit_links" table is now called "commit_edges".

There is now a "metrics" table that run measurements have a foreign key
to. This provides canonical metric metadata and will speed up the
initial graph page (before any real data arrives). For now, it will be
overwritten with each new run, but more nuanced config options may be
added later.

.sqlx/query-32f0ac59687e5455bb38060dc1d6cd181a86cd6ca7ac2f583a33212c8b7bef1a.json

@@ -0,0 +1,12 @@
+{
+  "db_name": "SQLite",
+  "query": "WITH RECURSIVE tracked (hash) AS ( SELECT hash FROM refs WHERE tracked UNION SELECT parent FROM commit_edges JOIN tracked ON hash = child ), reachable (hash) AS ( SELECT hash FROM refs UNION SELECT hash FROM tracked UNION SELECT parent FROM commit_edges JOIN reachable ON hash = child ) UPDATE commits SET reachable = CASE WHEN hash IN tracked   THEN 2 WHEN hash IN reachable THEN 1 ELSE 0 END ",
+  "describe": {
+    "columns": [],
+    "parameters": {
+      "Right": 0
+    },
+    "nullable": []
+  },
+  "hash": "32f0ac59687e5455bb38060dc1d6cd181a86cd6ca7ac2f583a33212c8b7bef1a"
+}