vcs-svn: simplify repo_modify_path and repo_copy

vcs-svn: simplify repo_modify_path and repo_copy

Restrict the repo_tree API to functions that are actually needed.

 - decouple reading the mode and content of dirents from other
   operations.
 - remove repo_modify_path.  It is only used to read the mode from
   dirents.
 - remove the ability to use repo_read_mode on a missing path.  The
   existing code only errors out in that case, anyway.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: David Barr <david.barr@cordelta.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>