vcs-svn: Implement Prop-delta handling

vcs-svn: Implement Prop-delta handling

The rules for what file is used as delta source for each file are not
documented in dump-load-format.txt.  Luckily, the Apache Software
Foundation repository has rich enough examples to figure out most of
the rules:

Node-action: replace implies the empty property set and empty text as
preimage for deltas.  Otherwise, if a copyfrom source is given, that
node is the preimage for deltas.  Lastly, if none of the above applies
and the node path exists in the current revision, then that version
forms the basis.

[jn: refactored, with tests]

Signed-off-by: David Barr <david.barr@cordelta.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>