Split from issue 4129.
The current implementation is sub-optimal: svn_fs_fs__verify will crawl
the whole repo with no UI feedback at all. Only after that will svnadmin
verify process the revisions and display some progress info.
The first phase takes about as long as the second, i.e. minutes to hours
for larger repos. If the work was done as part of the second stage, it
would be basically for free because of our internal caching (we access
the data anyway).
Also:
Verify does not detect the corruption in the example repository attached to
issue 4129