[BUG] git tab-completion gives error when "config" file exists in current directory

If a file "config" is present in the current directory, and no
.git/config or .gitconfig file exists, typing "git" and attempting tab
completion of any of its commands will yield an odd error like
"fatal: bad config file line 7 in config",
as git tries to use this file as a git config file.

Cheers,
Jeremy O'Brien