vcs-svn: Eliminate node_ctx.srcRev global

vcs-svn: Eliminate node_ctx.srcRev global

The srcRev variable is only used in handle_node(); its purpose
is to hold the old mode for a path, to only be used if properties
are not being changed.  Narrow its scope to make its meaningful
lifetime more obvious.

No functional change intended.  Add some tests as a sanity-check
for the simplest case (no renames).

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>