[rush] "rush change" fails to fetch origin/master in certain situations

[octogonz]

Repro steps:

1. Clone web-build-tools, then rollback your origin/master to commit hash e444346.
2. Now add a remote for raygesualdo:partial-prereleases from PR [rush] added partial prerelease functionality #1175.
3. Rewind the partial-prereleases branch to commit hash 8cee308
4. Run rush change

Expected: It should prompt for changes for the @microsoft/rush project only.
Actual: rush change wants change longs for ALL projects in the repo.

What's going on: The rush change command invokes VersionControl.getChangedFolders() which executes this command:

git diff origin/master... --dirstat=files,0

But it finds a huge diff because origin/master is not up to date. Before running this command, rush change was supposed to fetch changes using VersionControl._fetchNonDefaultBranch():

  private static _fetchNonDefaultBranch(remoteBranchName: string): void {
    // NOTE: DEFAULT_FULLY_QUALIFIED_BRANCH is "origin/master"
    if (remoteBranchName !== DEFAULT_FULLY_QUALIFIED_BRANCH) {
      console.log(`Checking for updates to ${remoteBranchName}...`);
      const fetchResult: boolean = VersionControl._tryFetchRemoteBranch(remoteBranchName);
      if (!fetchResult) {
        console.log(colors.yellow(
          `Error fetching git remote branch ${remoteBranchName}. Detected changed files may be incorrect.`
        ));
      }
    }
  }

However, the fetching is skipped because remoteBranchName === "origin/master". If I delete the if-statement, then the changes are fetched, the diff is correct, and rush change prompts for changes ONLY for the @microsoft/rush project.

(Thanks #1175 (comment) for reporting this!)

[octogonz]

@iclanton FYI since you worked on rush change recently.

[iclanton]

This has been fixed. I'll close this issue when it's released.