iago ran into this on IRC. "darcs what" knows that "bar" has been
removed, but we can't rename "foo" to "bar" until we explicitly tell
darcs that it's been removed. I'm not 100% sure this is a bug. The
behaviour seems inconsistent, but to fix it the "darcs mv" command would
need to add "remove foo" to _darcs/patches/pending, which seems a little
odd.

$ echo foo > foo
$ echo bar > bar
$ darcs init
$ darcs add foo bar
$ darcs rec -a -m initial
Finished recording patch 'initial'
$ rm bar
$ darcs what -s
R ./bar
$ darcs mv foo bar

darcs failed:  A file or dir named bar (or perhaps differing only in case)
already exists in repository.
Use --case-ok to allow files differing only in case.
$ darcs remove bar
$ darcs what -s
R ./bar
$ darcs mv foo bar
$ darcs --version
2.4.3 (release)
$