Use merge base instead pr base commit for file diff.

Author: rebornix

src/common/repository.ts

@@ -398,6 +398,15 @@ export class Repository {
 		let result = await this.run(['diff', '--no-ext-diff', '--exit-code']);
 		return result.exitCode !== 0;
 	}
+
+	async getMergeBase(a: string, b: string) {
+		let result = await this.run(['merge-base', a, b]);
+		if (result.exitCode === 0) {
+			return result.stdout.trim();
+		} else {
+			return null;
+		}
+	}
 }
 
 function parseRemote(remoteName: string, url: string): Remote | null {

src/github/interface.ts

@@ -131,6 +131,7 @@ export interface IPullRequestManager {
 	mayHaveMorePages(): boolean;
 	getPullRequestComments(pullRequest: IPullRequestModel): Promise<Comment[]>;
 	getPullRequestCommits(pullRequest: IPullRequestModel): Promise<Commit[]>;
+	getPullRequestMergeBase(pullRequest: IPullRequestModel): Promise<string>;
 	getCommitChangedFiles(pullRequest: IPullRequestModel, commit: Commit): Promise<FileChange[]>;
 	getReviewComments(pullRequest: IPullRequestModel, reviewId: string): Promise<Comment[]>;
 	getTimelineEvents(pullRequest: IPullRequestModel): Promise<TimelineEvent[]>;

src/github/pullRequestGitHelper.ts

@@ -223,4 +223,19 @@ export class PullRequestGitHelper {
 		let prConfigKey = `branch.${branchName}.${PullRequestMetadataKey}`;
 		await repository.setConfig(prConfigKey, PullRequestGitHelper.buildPullRequestMetadata(pullRequest));
 	}
+
+	static async getPullRequestMergeBase(repository: Repository, remote: Remote, pullRequest: IPullRequestModel): Promise<string> {
+		let mergeBase = await repository.getMergeBase(pullRequest.base.sha, pullRequest.head.sha);
+
+		if (mergeBase) {
+			return mergeBase;
+		}
+
+		const pullrequestHeadRef = `refs/pull/${pullRequest.prNumber}/head`;
+		await repository.fetch(remote.remoteName, pullrequestHeadRef);
+		await repository.fetch(remote.remoteName, pullRequest.base.ref);
+		mergeBase = await repository.getMergeBase(pullRequest.base.sha, pullRequest.head.sha);
+
+		return mergeBase;
+	}
 }

src/github/pullRequestManager.ts

@@ -236,6 +236,16 @@ export class PullRequestManager implements IPullRequestManager {
 		}
 	}
 
+	async getPullRequestMergeBase(pullRequest: IPullRequestModel): Promise<string> {
+		try {
+			const { remote } = await (pullRequest as PullRequestModel).githubRepository.ensure();
+			return PullRequestGitHelper.getPullRequestMergeBase(this._repository, remote, pullRequest);
+		} catch (e) {
+			vscode.window.showErrorMessage(`Fetching Pull Request merge base failed: ${formatError(e)}`);
+			return null;
+		}
+	}
+
 	async getCommitChangedFiles(pullRequest: IPullRequestModel, commit: Commit): Promise<FileChange[]> {
 		try {
 			const { octokit, remote } = await (pullRequest as PullRequestModel).githubRepository.ensure();

src/view/treeNodes/pullRequestNode.ts

@@ -137,7 +137,8 @@ export class PRNode extends TreeNode {
 			const comments = await this._prManager.getPullRequestComments(this.pullRequestModel);
 			const data = await this._prManager.getPullRequestChangedFiles(this.pullRequestModel);
 			await this._prManager.fullfillPullRequestCommitInfo(this.pullRequestModel);
-			const rawChanges = await parseDiff(data, this.repository, this.pullRequestModel.base.sha);
+			let mergeBase = await this._prManager.getPullRequestMergeBase(this.pullRequestModel);
+			const rawChanges = await parseDiff(data, this.repository, mergeBase || this.pullRequestModel.base.sha);
 			this._contentChanges = rawChanges.map(change => {
 				if (change instanceof SlimFileChange) {
 					return new RemoteFileChangeNode(