Add better help text to the repository form's Path field.

Our help text for the Path field was a big ol' mess of information for a
variety of SCMTools, and certainly didn't scale or look readable.

We now allow SCMTools to provide their own help text for any fields they
choose (though right now, it's only used for Path). When switching tools
on the repository form, the help text for all affected fields will
change.

The existing help text has been split up into the various SCMTools we
previously documented, and cleaned up a bit.

Switched around to various SCMTools. I saw what I expected on each one.