vcs-svn: Delay read of per-path properties

vcs-svn: Delay read of per-path properties

The mode for each file in an svn-format dump is kept in the properties
section.  The properties section is read as soon as possible to allow
the correct mode to be filled in when registering the file with the
repo_tree lib.

To support nodes with a missing properties section, svn-fe determines
the mode in three stages:

 - The kind (directory or file) of the node is read from the dump and
   used to make an initial estimate (040000 or 100644).
 - Properties are read in and allowed to override this for symlinks
   and executables.
 - If there is no properties section, the mode from the previous
   content of the path is left alone, overriding the above
   considerations.

This is a bit of a mess, and worse, it would get even more complicated
once we start to support property deltas.  If we could only register
the file with a provisional value for mode and then change it later
when properties say so, the procedure would be much simpler.

... oh, right, we can.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>