create_branch: move msg setup closer to point of use

create_branch: move msg setup closer to point of use

In create_branch() we write the reflog msg into a buffer in
the main function, but then use it only inside a
conditional. If you carefully follow the logic, you can
confirm that we never use the buffer uninitialized nor write
when it would not be used. But we can make this a lot more
obvious by simply moving the write step inside the
conditional.

Signed-off-by: Jeff King <redacted>