git-svn: enable logging of information not supported by git

The changes are now tracked in
  $GIT_DIR/svn/$GIT_SVN_ID/untracked.log

Information in the untracked.log include:
  * the addition and removal of empty directories
    (changes of these will also warn the user)
  * file and directory property changes, including (but not
    limited to) svk:merge and svn:externals
  * revision properties (revprops) are also tracked
  * users will be warned of 'absent' file and directories
    (if users are forbidden access)

Fields in entries are separated by spaces; "unsafe" characters
are URI-encoded so that each entry takes exactly one line.

There is currently no automated parser for dealing with the data
in untracked.log, but it should be possible to write one to
create empty directories on checkout and manage
externals/subprojects.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>