feat(review): add merge-base diff option for PR-style diffs (#485)

Author: yonihorn

packages/server/vcs.ts

@@ -72,7 +72,7 @@ export interface VcsProvider {
 
 // --- Git provider ---
 
-const GIT_DIFF_TYPES = new Set(["uncommitted", "staged", "unstaged", "last-commit", "branch"]);
+const GIT_DIFF_TYPES = new Set(["uncommitted", "staged", "unstaged", "last-commit", "branch", "merge-base"]);
 
 const gitProvider: VcsProvider = {
   id: "git",

packages/shared/review-core.ts

@@ -13,6 +13,7 @@ export type DiffType =
   | "unstaged"
   | "last-commit"
   | "branch"
+  | "merge-base"
   | `worktree:${string}`
   | "p4-default"
   | `p4-changelist:${string}`;
@@ -150,6 +151,7 @@ export async function getGitContext(
 
   if (currentBranch !== defaultBranch) {
     diffOptions.push({ id: "branch", label: `vs ${defaultBranch}` });
+    diffOptions.push({ id: "merge-base", label: `Current PR Diff` });
   }
 
   const [worktrees, currentTreePathResult] = await Promise.all([
@@ -385,6 +387,28 @@ export async function runGitDiff(
         break;
       }
 
+      case "merge-base": {
+        const mergeBaseResult = assertGitSuccess(
+          await runtime.runGit(["merge-base", defaultBranch, "HEAD"], { cwd }),
+          ["merge-base", defaultBranch, "HEAD"],
+        );
+        const mergeBase = mergeBaseResult.stdout.trim();
+        const mergeBaseDiffArgs = [
+          "diff",
+          "--no-ext-diff",
+          `${mergeBase}..HEAD`,
+          "--src-prefix=a/",
+          "--dst-prefix=b/",
+        ];
+        const mergeBaseDiff = assertGitSuccess(
+          await runtime.runGit(mergeBaseDiffArgs, { cwd }),
+          mergeBaseDiffArgs,
+        );
+        patch = mergeBaseDiff.stdout;
+        label = `PR diff vs ${defaultBranch}`;
+        break;
+      }
+
       default:
         return { patch: "", label: "Unknown diff type" };
     }
@@ -474,6 +498,14 @@ export async function getFileContentsForDiff(
         oldContent: await gitShow(defaultBranch, oldFilePath),
         newContent: await gitShow("HEAD", filePath),
       };
+    case "merge-base": {
+      const mbResult = await runtime.runGit(["merge-base", defaultBranch, "HEAD"], { cwd });
+      const mb = mbResult.exitCode === 0 ? mbResult.stdout.trim() : defaultBranch;
+      return {
+        oldContent: await gitShow(mb, oldFilePath),
+        newContent: await gitShow("HEAD", filePath),
+      };
+    }
     default:
       return { oldContent: null, newContent: null };
   }