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.

See early comments on this issue [below this commit](https://github.com/alerque/vcsh/commit/bae3226ff27b0fb0aa096b87f6426e3df927aeec#commitcomment-3273470). The case of the script bombing without having dealt with the clobbers and hence leaving half done files around is still not taken care of -- they will continue to be potentially lost, but the hackery involved is moved out of the core code and into a hook script.