vcs-svn: introduce repo_read_path to check the content at a path

vcs-svn: introduce repo_read_path to check the content at a path

The repo_tree structure remembers, for each path in each revision, a
mode (regular file, executable, symlink, or directory) and content
(blob mark or directory structure).  Maintaining a second copy of all
this information when it's already in the target repository is
wasteful, it does not persist between svn-fe invocations, and most
importantly, there is no convenient way to transfer it from one
machine to another.  So it would be nice to get rid of it.

As a first step, let's change the repo_tree API to match fast-import's
read commands more closely.  Currently to read the mode for a path,
one uses

	repo_modify_path(path, new_mode, new_content);

which changes the mode and content as a side effect.  There is no
function to read the content at a path; add one.

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>