1. Summarise the issue (what were doing, what went wrong?)

I was running "darcs changes --repo-dir". It reported that it was "not a 
repository". but I knew it existed. 

Here is a more complete test case syntax:

darcs changes -a --max-count=1 --repo=user@remote.com:/home/user/repo 
./file/path.txt

And a more complete diagnostic output:

darcs failed:  Not a repository: user@remote.com:/home/user/repo  (.: 
openBinaryTempFile: permission denied (Permission denied))

From the diagnostic, I could guess what's happening. The problem is really 
that there is a "permission denied" in the local directory of ".", and the 
diagnostic message is sub-optimal. 

I expect the "darcs changes" command to be read-only, so it's suprising 
that needs write-access locally. (What is it writing?!)

The workaround is also add a --repodir argument to provide a local darcs 
directory that the current user can write to. 

3. What darcs version are you using? (Try: darcs --exact-version)

darcs 2.8.0

4. What operating system are you running?
 
Ubuntu Linux.