repo-config needs a prefix.

This fixes the message received when invoking certain commands from 
outside of a git tree, so e.g. instead of receiving this:
/home/rob/bin/git-fetch: line 89: /FETCH_HEAD: Permission denied
We get this again:
fatal: Not a git repository: '.git'
---
  git.c |    2 +-
  1 files changed, 1 insertions(+), 1 deletions(-)