Second-draft SVN revprop change handling

Fix #3100: Left out a case (a change to an issue that a change set is
already associated with) which this change fixes. There’s a logging
message included just so I can keep an eye on it.

Also changes for default values in custom fields not being recorded when
auto-fixing an issue (such as by the SI plugin).

Signed-off-by: Murray Crane <murray.crane@gmail.com>