How to handle terminal detection in a daemon calling git?

I've got a script that runs in the background without a terminal. It actually does have STDOUT and STDERR set to a rotating log file.

When it runs:
  /bin/sh -c 'cd /to/my/wc; git pull --ff-only'
the git command fails (rc 32768).

When it runs
 /bin/sh -c 'cd /to/my/wc; git pull --ff-only > /to/a/file 2>&1'

or even
 /bin/sh -c 'cd /to/my/wc; git pull --ff-only | cat'

then all is well. The command succeeds (rc 0, and I see the expected results).

Piping through 'cat' is okay, but I'd rather avoid the 'trick'. Is there some way to communicate to git that it should operate just as if output were redirected?

-Travis