Funny, because 'svn log -rBASE:HEAD' works fine.
Look at libsvn_client/diff.c:do_diff(). It attempts to pigeonhole the
inputs into one of three scenarios: wc vs. wc, wc vs. repos, or
repos vs. repos.
The bug is that it's failing to recognize that a comparison between
BASE and HEAD falls into the 3rd category. It's treating the BASE
concept as if it were a working-copy thing, when it *should* interpret
the keyword as a file's working revnum...(i.e. the revnum that the
text-base represents.) Then it would have two specific server-side
revnums to compare.