When running arc patch command on Windows command line with SVN repository, the command fails with following message:
C:\working_copy\arc patch Dxx The system cannot find the path specified. WARNING Some hunks could not be applied cleanly by the unix 'patch' utility. Your working copy may be different from the revision's base, or you may be in the wrong subdirectory. You can export the raw patch file using 'arc export --unified', and then try to apply it by fiddling with options to 'patch' (particularly, -p), or manually. The output above, from 'patch', may be helpful in figuring out what went wrong.
The command fails because of ; characters in command chaining (e.g. (cd %s; patch -p0 < %s)) as was predicted in this todo comment in ArcanistPatchWorkflow.php.
I made a quick fix in my github fork so that may shed some more light on the issue if needed. Basically it uses setCWD() method on future/passthru instead of directly calling cd.