SVN import: Use one log call

One "svn log" (or its equivalent) per revision adds delay and server load.
Instead, open two SVN connections -- one for the log, and one for the files.

Positive side effect: Only those log entries which actually contain data
are committed => no more empty commits.

Also, change the "-l" option to set the maximum revision to be pulled,
not the number of revisions.

Signed-off-by: Junio C Hamano <junkio@cox.net>