add exportedtree to info

info: When used with an exporttree remote, includes an "exportedtree" info,
which is the tree last exported to the remote. During an export conflict,
multiple values will be listed.

This commit was sponsored by John Pellman on Patreon.

doc/todo/add_prefix_option_to_export/comment_5_fe7223a5321b8a4aef723e276513f5be._comment

@@ -0,0 +1,23 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 5"""
+ date="2018-12-03T17:53:59Z"
+ content="""
+I wonder if you'd be better off querying `git annex whereis --json` 
+for public urls and providing those to the users. Several special remotes
+provide public urls. (S3 needs a non-default configuration to do it.)
+
+Anyway back to your question, git-annex arranges for the exported tree to
+always be available, including across clones of the repository. So you can
+get the exported tree, graft another file into it, and export the new tree.
+
+So how to look up the previously exported tree? [[/internals]] documents
+how to find it in the export.log, but it seems there ought to be a
+command-line interface to query for it. So I've made `git annex info remote`
+provide that information, as "exportedtree". Note that in an export
+conflict it may have multiple values. You'll want to use --fast with that,
+and probably --json.
+
+Let me know if you need something more, or if this todo can be closed with
+that.
+"""]]