vcs-svn: avoid unnecessary copying of log message and author

vcs-svn: avoid unnecessary copying of log message and author

Use strbuf_swap when storing the svn:log and svn:author properties, so
pointers to rather than the contents of buffers get copied.  The main
effect should be to make the code a little easier to read.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>