When creating branch c/d check that branch c does not already exists.

When creating branch c/d check that branch c does not already exists.

With packed refs, there may not be a ".git/refs/heads/c" file
when branch c exists. And currently in this case, there is no check
to prevent creation of branch c/d.

This should probably be rewritten in C and done after the ref lock
has been taken to make sure no race exists though.

This is mainly to make all test cases in "t3210-pack-refs.sh" work.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>