From: martin f. krafft <madduck@debian.org>
Subject: [TopGit PATCH] Ensure we are inside a Git worktree

Bert Wesarg suggests to check that we are not inside the .git directory before
setting up topgit. Because git rev-parse --git-dir only works in the top .git
dir, not deeper (i.e. inside .git/refs) and will always return "."

This patch thus causes topgit to die early on when it's called from outside of
a Git repository, or from underneath .git and outputs error messages
accordingly.

This temporarily makes it even more impossible to call tg --help, but I expect
to have that fixed soon too.

Signed-off-by: martin f. krafft <madduck@debian.org>