Updated patch series for providing mechanism to list available repositories

I have updated the patch series sent to this list on July 13th.  Based
largely on feedback from this list, I have made the following changes:

- sample commands now live in contrib/git-shell-commands
- blank lines have been added to patch 1's commit message for readability
- commands are run with cwd of the user's $HOME directory
- run_command is used rather than writing a new function with the same functionality
- the shell's loop resides in its own function
- commands are now parsed with split_cmdline, so helper functions can be passed arguments

Thanks to those who provided feedback.  I look forward to further
comments!

Greg