Fix temporaryDeploymentBranch conflict by replacing slash with hyphen

[Copilot]

This PR fixes a Git reference conflict that occurs when creating temporary deployment branches. The action was failing when a branch named github-pages-deploy-action already exists in the repository.

Problem

The action creates temporary branches using the pattern:

github-pages-deploy-action/{random-string}

When a branch named github-pages-deploy-action already exists, Git cannot create the temporary branch because it interprets branch names with forward slashes as nested references. This causes the error:

fatal: cannot lock ref 'refs/heads/github-pages-deploy-action/sh2ajsxux': 'refs/heads/github-pages-deploy-action' exists; cannot create 'refs/heads/github-pages-deploy-action/sh2ajsxux'

Solution

Changed the temporary branch naming pattern from:

`github-pages-deploy-action/${randomString}`

To:

`github-pages-deploy-action-${randomString}`

This uses a hyphen separator instead of a forward slash, eliminating the Git reference conflict while maintaining a clear naming convention.

Changes Made

• src/git.ts: Modified line 111 to use hyphen instead of forward slash in temporaryDeploymentBranch
• tests/git.test.ts: Added test case to validate branch name format and prevent regression

Testing

• All existing tests pass (60/60)
• New test validates that temporary branch names use hyphens and don't contain forward slashes
• Build and linting pass without issues

This minimal change resolves the conflict without affecting any other functionality or breaking existing workflows.

Fixes #1712.

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