vcs-svn: use strbuf for author, UUID, and URL

vcs-svn: use strbuf for author, UUID, and URL

Use strbufs and strings instead of interned strings for values of rev,
dump, and node fields that happen to be strings.  After this change,
the only remaining string_pool use is for paths in the repo_tree API
and internals.

Functional change: treat an empty author, UUID, or URL as none at all.
So for example, in repos where the first revision has an empty
svn:author property, the first rev will be treated as by "nobody"
rather than by a person with empty name and email address created by
prepending an @ sign to the repository UUID.

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