OS X darcs 2.10.1 diff: darcs: /usr/bin/diff: fileAccess: permission denied (Operation not permitted)

I just wrote up this bug but then discovered it doesn't affect 2.12,
so maybe it's not worth chasing down.  Still though, 2.10.1 is the
latest version with an OS X binary download, and compiling darcs
yourself involves downloading a huge amount of haskell infrastructure,
so I'd guess OS X people will mostly be on 2.10.  And of course 2.12
has that slow whatsnew so for me at least 2.10 is better, except for:

Here's another darcs bug that's come up:

Lately every time I use 'darcs diff' I get this error.  I tried
putting on all the verbose flags I could and it still won't tell me
what it's actually trying to run:

% d diff --last=1 --debug --verbose --timings
Wed Aug  2 18:10:40 PDT 2017: Beginning identifying repository .
Wed Aug  2 18:10:40 PDT 2017: Done identifying repository .
Wed Aug  2 18:10:40 PDT 2017: Identified darcs-2 repo:
/Users/elaforge/src/seq/solkattu-group
Wed Aug  2 18:10:41 PDT 2017: Reading patch file: patch
82a516548d00143cb282ff9b2d54b72e5f708f21
Author: [email protected]
Date:   Wed Aug  2 15:40:36 PDT 2017
  * solkattu: extract tempo to its own argument
Wed Aug  2 18:10:41 PDT 2017: I'm doing copyFileUsingCache on
patches/0000000858-246ed887d585d50bbdeb0f3e2036e01ccd1baa8a3b007af3efadfdfd41e520be
Wed Aug  2 18:10:41 PDT 2017: About to gzFetchFilePS from
"/Users/elaforge/src/seq/solkattu-group/_darcs/patches/0000000858-246ed887d585d50bbdeb0f3e2036e01ccd1baa8a3b007af3efadfdfd41e520be"
Wed Aug  2 18:10:41 PDT 2017: gzFetchFilePS done.
darcs: /usr/bin/diff: fileAccess: permission denied (Operation not permitted)

--store-in-memory doesn't help.  Even --diff-command='echo %1 %2'
gives the same result.  /usr/bin/diff exists, and has permissions
0755, and works fine, so I don't know who exactly is getting that
error.


Probably the best course is to fix the slow whatsnew, and then update
the OS X binary on http://darcs.net/Binaries