feat: Add git branch functionality and unit tests

This commit introduces the `git branch` tool to the MCP Git server, allowing users to list branches with various filtering options.

Changes include:
- Implemented `git_branch` function in `src/git/src/mcp_server_git/server.py` to support listing local, remote, and all branches, as well as filtering by `contains` and `not_contains` SHA values.
- Added comprehensive unit tests for the `git branch` functionality in `src/git/tests/test_server.py`, covering different branch types and commit filtering scenarios.
- Updated `src/git/README.md`.

src/git/README.md

@@ -16,14 +16,16 @@ Please note that mcp-server-git is currently in early development. The functiona
 
 2. `git_diff_unstaged`
    - Shows changes in working directory not yet staged
-   - Input:
+   - Inputs:
      - `repo_path` (string): Path to Git repository
+     - `context_lines` (number, optional): Number of context lines to show (default: 3)
    - Returns: Diff output of unstaged changes
 
 3. `git_diff_staged`
    - Shows changes that are staged for commit
-   - Input:
+   - Inputs:
      - `repo_path` (string): Path to Git repository
+     - `context_lines` (number, optional): Number of context lines to show (default: 3)
    - Returns: Diff output of staged changes
 
 4. `git_diff`
@@ -31,6 +33,7 @@ Please note that mcp-server-git is currently in early development. The functiona
    - Inputs:
      - `repo_path` (string): Path to Git repository
      - `target` (string): Target branch or commit to compare with
+     - `context_lines` (number, optional): Number of context lines to show (default: 3)
    - Returns: Diff output comparing current state with target
 
 5. `git_commit`
@@ -297,12 +300,13 @@ If you are doing local development, there are two ways to test your changes:
 "mcpServers": {
   "git": {
     "command": "uv",
-    "args": [ 
+    "args": [
       "--directory",
       "/<path to mcp-servers>/mcp-servers/src/git",
       "run",
       "mcp-server-git"
     ]
+    }
   }
 }
 ```