hc: Check if upgrade is actually needed before doing it
hc: Implement uninstall command
hc: Make find backward-compatible

It's mainly used for finding bare files in repos, which will go away
eventually (maybe?).  But for now, be compatible.
hc: Extend the 'path' command to update env vars

Teach 'hc path' to merge hook paths with an existing environment
variable.  If the intention is to use 'hc path' to update an environment
variable, 'hc path' can now be used to re-calculate the value of the
environment variable, removing duplicates.