Expose `Graph.mergeManyConservative` (#209)

Summary:
This offers #205 to general users.

Test Plan:
Existing tests suffice.

wchargin-branch: merge-many-conservative

src/core/graph.js

@@ -338,6 +338,28 @@ export class Graph<NP, EP> {
     );
     return result;
   }
+
+  /**
+   * Equivalent to
+   *
+   *     graphs.reduce((g, h) => Graph.mergeConservative(g, h), new Graph()),
+   *
+   * but uses a mutable accumulator for improved performance.
+   */
+  static mergeManyConservative<NP, EP>(
+    graphs: $ReadOnlyArray<Graph<$Subtype<NP>, $Subtype<EP>>>
+  ): Graph<NP, EP> {
+    const result = new Graph();
+    graphs.forEach((graph) => {
+      graph.nodes().forEach((node) => {
+        result.addNode(node);
+      });
+      graph.edges().forEach((edge) => {
+        result.addEdge(edge);
+      });
+    });
+    return result;
+  }
 }
 
 export function edgeID(src: Address, dst: Address): string {

src/plugins/git/createGraph.js

@@ -41,7 +41,7 @@ class GitGraphCreator {
     const treeAndNameToSubmoduleUrls = this.treeAndNameToSubmoduleUrls(
       repository
     );
-    const graphs = [
+    return Graph.mergeManyConservative([
       ...Object.keys(repository.commits).map((hash) =>
         this.commitGraph(repository.commits[hash])
       ),
@@ -49,17 +49,7 @@ class GitGraphCreator {
         this.treeGraph(repository.trees[hash], treeAndNameToSubmoduleUrls)
       ),
       this.becomesEdges(repository),
-    ];
-    const result = new Graph();
-    graphs.forEach((g) => {
-      g.nodes().forEach((node) => {
-        result.addNode(node);
-      });
-      g.edges().forEach((edge) => {
-        result.addEdge(edge);
-      });
-    });
-    return result;
+    ]);
   }
 
   treeAndNameToSubmoduleUrls(repository: Repository) {