Allow reading svn dumps from files via file:// urls

Allow reading svn dumps from files via file:// urls

For testing as well as for importing large, already available dumps,
it's useful to bypass svnrdump and replay the svndump from a file
directly.

Add support for file:// urls in the remote url, e.g.

  svn::file:///path/to/dump

When the remote helper finds an url starting with file:// it tries to
open that file instead of invoking svnrdump.

Signed-off-by: Florian Achleitner <redacted>
Acked-by: David Michael Barr <redacted>
Signed-off-by: Junio C Hamano <redacted>