convert/svn: do not try converting empty head revisions (
issue3347)
Subversion conversion works by picking trunk and branches heads, computing a
revision graph from them and converting the selected commits. By design we fail
to convert empty revisions so we have to be careful when discovering the
revision graph. In this particular issue, the source svn repository was a
partial mirror made by svnsync. The funny part is svnsync preserves all
revisions including empty ones. Also, we trusted ra.stat(path,
stop).created_rev to give us the latest revision with changes in path history
up to stop. This assumption broke at least when path is '', that is the
repository root, which always returned 'stop' revision despited being empty.
The workaround is to first trust ra.stat() but if the returned revision appear
empty, search the whole path history from stop to r1 until some changes are
found.