check if it's under a git repo before running `git remote -v`

there was error message from `git remote -v` if you try to fork a repo
under a non-git-repo directory:
$ code fork codecli
fatal: Not a git repository (or any of the parent directories): .git
usage: code fork [-h] [-v] [-p PROVIDER] upstream origin [dir]
code fork: error: too few arguments
Merge pull request #38 from xupeng/codecli:check-git-repo

check if it's under a git repo before running `git remote -v`