hooks: move the vcs hook entry points and setup code out of lib

Mercurial hooks are running in a process that already has been initialized, so
they invoke the hooks lib directly. Git hooks are binaries and need a lot of
initialization before they can do the same. Move this extra setup code
elsewhere.

Having this high level code in bin is perhaps also not ideal, but it also
doesn't seem that bad: that is where other command line entry points invoke
make_app.

(It seems like it could be adventageous to somehow use "real" bin commands for
hooks ... but for now we use the home-made templates.)

Note: As a side effect of this change, all git hooks *must* be re-installed
when upgrading.