git-init: accept --bare option

git-init: accept --bare option

It is unfortunate that "git init --bare" does not work and the only reason
why "init" did not learn its own "--bare" option is because "git --bare
init" already does the job (and as an option to the git 'potty', it is
more generic solution).

This teaches "git init" its own "--bare" option, so that both "git --bare init"
and "git init --bare" works mostly the same way.

[jc: rewrote the log message and added test]

Signed-off-by: Luciano Rocha <strange@nsk.pt>
Signed-off-by: Junio C Hamano <gitster@pobox.com>