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>