Qualify default branch ref in ignore_revs_file_blob

Gavin Hinfeyrequested to merge
fix-ignore-revs-blob-dash-default-branch into master

What does this MR do and why?

Repository#ignore_revs_file_blob passed the bare default branch name as the Gitaly revision. When the branch name starts with - (possible via import, mirror, or update-ref), Gitaly's revision validation rejects it and the raw GRPC::InvalidArgument (revision can't start with '-') escapes into the repository tree view render, causing a 500.

This MR qualifies the revision as refs/heads/#{project.default_branch}, matching the existing call sites for the same file in Gitlab::Blame and Resolvers::BlameResolver. A no-op for valid branch names.

How to set up and validate locally

  1. Create a project and set a dash-prefixed default branch directly in the repo, e.g. git update-ref refs/heads/-foo <sha> + git symbolic-ref HEAD refs/heads/-foo.
  2. Visit the project's repository tree page.
  3. Before: 500 (GRPC::InvalidArgument). After: page renders normally.

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Edited by Gavin Hinfey

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