We generate a reflog message that contains some fixed text
plus a branch name, and use a buffer of size PATH_MAX + 20.
This mostly works if you assume that refnames are shorter
than PATH_MAX, but:
1. That's not necessarily true. PATH_MAX is not always the
filesystem's limit.
2. The "20" is not sufficiently large for the fixed text
anyway.
Let's just switch to a heap buffer so we don't have to even
care.