BinSrc.GitUtils - add `in_git_repo` function

A function to detect whether we are in a git repository

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
Create `git-switch-branch` program

git-switch-branch presents an simple, interactive interface that prints all
available git branches in the current git repository, and asks the user which
branch to switch to. Instead of entering the full branch name, the user enters
an index for the branch to switch to.

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>