Disable svn:entry:committed-date

Disable the setting of the svn:entry:committed-date property as this
can interfere with repository migration
Use svn_client_create_context() for getting SVN context

svn_client_ctx_t() no longer seems to work with newer versions of the
python Subversion libraries, so switch to
svn_client_create_context().  Also, while here:

 - When transforming SCCS keywords, commit every 256 files to avoid
   the infamous 'Too many open files' errors
 - Minor change to keyword transformation commit message
Make keyword conversion optional

Make conversion of keywords optional