This is probably a "not worth fixing", but I noticed that Darcs seems
to just *ASSUME* $HOME is an absolute path.  Couple with its use of
chdir(2) to the repo root before doing anything, this results in a
failure:

    $ with-temp-dir sh -c 'HOME=. darcs get http://code.haskell.org/yi'
    with-temp-dir: entering directory `/tmp/with-temp-dir.x6ukO5'
    darcs:
    ./.darcs/cache/pristine.hashed/0000000931-848fba01e3390b4e9ac197f3712bd018b592a61b13be6180a3469957d1ea08e8-new_c2c8459d74b10fb739a6f62f34ff2245:
    renameFile: does not exist (No such file or directory)
    [...]

If I use $PWD instead of ., it works:

    $ with-temp-dir sh -c 'HOME=$PWD darcs get http://code.haskell.org/yi'
    with-temp-dir: entering directory `/tmp/with-temp-dir.QIqjJL'
    Copying patches, to get lazy repository hit ctrl-C...
    Copying patches 55/4117
    [...]

I'll declare this a wont-fix unless somebody really wants to fight for it!