Check if user is in a repo before `feature list` starts.

This prevents `feature list` from listing a bunch of error messages:

```
rchoudhury@cominor ~
> feature list                                                                                                                         
fatal: Not a git repository (or any parent up to mount point /mnt/ebs)
Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).
fatal: Not a git repository (or any parent up to mount point /mnt/ebs)
Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).

Current Branch:
------------------------------------------------------------
fatal: Not a git repository (or any parent up to mount point /mnt/ebs)
Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).
(not on any branch!)

Available feature branches:
------------------------------------------------------------
(none)
fatal: Not a git repository (or any parent up to mount point /mnt/ebs)
Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).
/mnt/ebs/home/ifixit/git-scripts/lib/helpers.rb:174:in `initialize': Permission denied - /git-scripts.log (Errno::EACCES)
        from /mnt/ebs/home/ifixit/git-scripts/lib/helpers.rb:174:in `open'
        from /mnt/ebs/home/ifixit/git-scripts/lib/helpers.rb:174:in `log_command'
        from /mnt/ebs/bin/feature:260:in `<main>'
```