vcs-svn: Implement prop-delta handling.

vcs-svn: Implement prop-delta handling.

By testing against the Apache Software Foundation
repository, some simple rules for decoding prop
deltas were derived.

'Node-action: replace' implies the empty prop set
as the base for the delta.
Otherwise, if a copyfrom source is given that node
forms the basis for the delta.
Lastly, if the destination path exists in the active
revision it forms the basis.

The same rules ought to apply to text deltas as well.

Apply these rules to prop handling.
Add a placeholder srcMark parameter to fast_export_blob().

Signed-off-by: David Barr <david.barr@cordelta.com>