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(-)

This time with Signed-off-by line.

index 18ba14a..f9c76a1 100644
@@ -264,7 +264,7 @@ static void handle_internal_command(int 
 		{ "prune", cmd_prune, NEEDS_PREFIX },
 		{ "mv", cmd_mv, NEEDS_PREFIX },
 		{ "prune-packed", cmd_prune_packed, NEEDS_PREFIX },
-		{ "repo-config", cmd_repo_config },
+		{ "repo-config", cmd_repo_config, NEEDS_PREFIX },
 	};
 	int i;