Specify custom Git executable in options and locally in batchfetch tool

[jamescherti]

The task involves adding the ability to specify the Git executable in the options section of the batchfetch configuration file as well as locally within the environment. This feature should allow users to define a custom Git executable path, enabling the tool to use a specific version of Git or one located in a non-standard directory.

---

options:
  git_executable: /opt/local/bin/git