is_refname_available(): use dirname in first loop

is_refname_available(): use dirname in first loop

In the first loop (over prefixes of refname), use dirname to keep
track of the current prefix. This is not an improvement in itself, but
in a moment we will start using dirname for a role where a
NUL-terminated string is needed.

Signed-off-by: Michael Haggerty <redacted>