fs-rep-sharing infinite loop (again)

In working on the fs-rep-sharing branch, I've stumbled into a problem with bdb
post-commit deltification. When loading r2555 of the Subversion repository, I
get what appears to be an infinite loop in read_rep_range() in
subversion/libsvn_fs_base/reps-strings.c, specifically in the inner loop which
collects reps needed to perform undeltifiacation. This occurs when processing
/trunk/subversion/libsvn_wc/log.c.

I'm sending this here so that other interested parties can take a look, and so
that I don't forget the result of my analysis.

-Hyrum