Add hooks to allow fancy merge handling

I ripped out my initial attempt at setting up an unclobber system that would allow cloning repositories into dirty space where some tracked files already existed and moved the hackery to a pair of hook scripts. The original code that detects possible conflicts is back in place, but two new hook locations allow these conflicts to be actually handled before the script detects trouble ahead and bombs out.

I have included a set of sample hooks that does the job with a simple unclobber method that allows the repository to clone cleanly but leaves it with the working copy with unstaged changes that are the local versions.