fix(git): create a branch from a detached HEAD

[kabishou11]

Description

git_create_branch without base_branch fails whenever the repository has a detached HEAD (after checking out a commit or tag, or in the middle of a rebase). It uses repo.active_branch as the start point, and GitPython raises TypeError: HEAD is a detached symbolic reference as it points to '<sha>' when there is no active branch. Creating a branch from a detached HEAD is exactly how you keep that work (git switch -c <name>), so the tool fails on a normal workflow.

This change branches from the current commit when HEAD is detached. The result message reports the commit SHA. Behaviour with a base_branch, or on a normal branch, is unchanged.

Server Details

• Server: git
• Changes to: tools (git_create_branch)

Motivation and Context

repo.git.checkout(repo.head.commit.hexsha)    # detach HEAD
git_create_branch(repo, "rescued-branch")
# before: TypeError: HEAD is a detached symbolic reference as it points to '1c2a66c...'
# after:  Created branch 'rescued-branch' from '1c2a66c...'

How Has This Been Tested?

• New test test_git_create_branch_detached_head. It fails on main with the TypeError and passes with this change.
• uv run pytest in src/git: 48 passed.
• uv run pyright src: 0 errors. ruff check: clean.

Breaking Changes

None.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Protocol Documentation
• My changes follows MCP security best practices
• I have updated the server's README accordingly (not needed)
• I have tested this with an LLM client
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have documented all environment variables and configuration options (none added)

Additional context

#4943 changes the same function for annotated tags given as base_branch. That is a different code path and the two changes don't overlap.

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