new/init: default git branch to 'main'; update tests and Dockerfiles

[doocho]

Summary

• Change: cargo new/cargo init initializes Git repos with main as the default branch.
• Update: Tests and test Dockerfiles adjusted to match the new default.

Motivation

• Consistency: Stable behavior regardless of user init.defaultBranch.
• Convention: Align with current industry standard and inclusive naming.

Behavior change

• Before: Default branch could be master or follow user Git config.
• After: Always main for repos initialized by Cargo. Existing repos unaffected.

Implementation details

• Core: Set libgit2 RepositoryInitOptions::initial_head("main") in GitRepo::init.
• Files:
  • src/cargo/util/vcs.rs
  • tests/testsuite/new.rs
  • crates/cargo-test-support/containers/apache/Dockerfile
  • crates/cargo-test-support/containers/sshd/Dockerfile

Tests

• Adjusted: tests/testsuite/new.rs expects refs/heads/main.
• Infra: Test Dockerfiles use git init -b main.

Backwards compatibility

• Scope: Only affects newly created repos via cargo new/cargo init.
• Note: Scripts assuming master on fresh repos should be updated.

How to verify

cargo new foo
git -C foo symbolic-ref HEAD  # expects: refs/heads/main

Alternatives considered

• Respect init.defaultBranch: Rejected to ensure deterministic behavior across environments and CI.
• Config override: Could be added later if configurability is desired.

Release notes

• Changed: cargo new/cargo init now create Git repositories with main as the default branch.

Checklist

• Code updated
• Tests updated
• Test Dockerfiles updated
• Documentation (can update if there’s a canonical place describing default branch behavior)

[rustbot]

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[weihanglo]

Cargo delegates this to libgit2, so it respects whatever libgit2 respects. I would generally recommend we don't do this change in this PR. We should discuss in an issue first before proceeding, per our contributing guideline

Other parts changing dockerfiles and tests are fine, btw, as they are not user-facing.

See also:

• Change default git branch name to Main instead of Master #8926
• Default git branch name is master not main on MacOS #14650

[epage]

Based on at, I'm going to go ahead and close this.