We have two cases that have come up semi-frequently, tripping up users

when they're trying to configure their repositories:

Users were putting in a raw file path to an SVN repository, where it

   needs a file:// URL.
When configuring a Git repo, they'd enter their SVN clone address but

   leave out the raw file URL. This one is especially confusing because

   a lot of people expect Git to just work, and don't read our

   documentation about why it doesn't.

This change adds new repository form subclasses that can validate these

conditions.

Verified that I wasn't able to save an SVN repo pointing to a bare

  filesystem path, but could with file:// and https://
Verified that trying to save a Git repo with a path but no raw file

  URL would fail.
Made sure that saving with a raw file URL or Git repos on a hosting

  service still worked correctly.
Ran unit tests.