Plugins create dangling edges

The GitHub plugin no longer adds a Node to the graph for Git commits.
Instead, it creates a dangling edge to it. This frees the GitHub plugin
from responsibility for setting the timestamp or other metadata for Git
nodes.

The Git plugin no longer adds a Commit Node to the graph immediately when
encountering a commit's parent hash. Instead, it creates an edge to the
parent, and then fills in the parent node once it is encountered in the
commit store.

Test plan: Load a real repository with merged pull requests
(e.g. sourcecred/research) into the explorer, and verify that GitHub
commit entities are still connected to Git commits, and that Git
commits are still connected to their parents.

src/plugins/git/createGraph.js

@@ -34,7 +34,6 @@ class GraphCreator {
     this.graph.addNode({address: GN.toRaw(node)});
     for (const parentHash of commit.parentHashes) {
       const parent: GN.CommitAddress = {type: GN.COMMIT_TYPE, hash: parentHash};
-      this.graph.addNode({address: GN.toRaw(parent)});
       this.graph.addEdge(GE.createEdge.hasParent(node, parent));
     }
   }

src/plugins/github/__snapshots__/createGraph.test.js.snap

@@ -2560,24 +2560,6 @@ Array [
       },
     ],
     "nodes": Array [
-      Object {
-        "index": 0,
-      },
-      Object {
-        "index": 1,
-      },
-      Object {
-        "index": 2,
-      },
-      Object {
-        "index": 3,
-      },
-      Object {
-        "index": 4,
-      },
-      Object {
-        "index": 5,
-      },
       Object {
         "index": 6,
       },

src/plugins/github/createGraph.js

@@ -45,8 +45,6 @@ class GraphCreator {
         type: GitNode.COMMIT_TYPE,
         hash: commit.hash(),
       };
-      const gitCommit = GitNode.toRaw(gitCommitAddress);
-      this.graph.addNode({address: gitCommit});
       this.graph.addEdge(
         createEdge.correspondsToCommit(commit.address(), gitCommitAddress)
       );