fix reflog entries for "git-branch"

fix reflog entries for "git-branch"

Even when -l is not given from the command line, the repository
may have the configuration variable core.logallrefupdates set,
or an old-timer might have done ": >.git/logs/refs/heads/new"
before running "git branch new".  In these cases, the code gave
an uninitialized msg[] from the stack to be written out as the
reflog message.

This also passes a different message when '-f' option is used.
Saying "git branch -f branch some-commit" is a moral equilvalent
of doing "git-reset some-commit" while on the branch.

Signed-off-by: Junio C Hamano <junkio@cox.net>