Fix detached checkout of non-default dependency branches

[Shubham-Padkonde]

Non-default branches in Git repository dependencies fail with fatal: --detach cannot be used with -b/-B/--orphan: after cloning, the branch exists only as a remote-tracking reference, and Git's automatic local branch creation conflicts with detached checkout.

Resolve an existing commit reference first, falling back to refs/remotes/origin/<reference> when it is absent. This preserves default-branch, commit-hash, and tag behavior, including a tag sharing its name with a remote branch.

The regression creates a local Git repository and runs a real DUB consumer against four references, checking the dependency's compiled value. The non-default branch fails before the fix; all four cases pass afterward. The fixture uses isolated temporary repositories and package caches, without a network dependency.

Validation on Linux, LDC 1.43.0:

• ldc2 -run build.d with DMD=ldmd2: passed.
• test/issue3047-git-branch.sh: all four consumer builds/runs passed.
• dub test --compiler=ldc2 --skip-registry=all -c library: 59 modules passed.
• Shell syntax and git diff --check: passed.

Fixes #3047 for the dependency checkout failure. The separately reported Windows cleanup/access-denied path has not been tested or changed. Full OS/compiler integration matrix not run.

Developed and tested with OpenAI Codex assistance; no independent human review is claimed.

[github-actions]

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[Shubham-Padkonde]

Buildkite 4215 stopped before compilation because merge_head.sh could not fetch refs/pull/3162/merge (exit 128). That reference now resolves to 9834ba1e43e93efa9d1856c56f96b83c1aa5731f. Could someone with Buildkite access rerun it? This is the same initial checkout failure seen on #3161; no source changes were made for it. Local validation and agent assistance are disclosed above.