verify_refname_available(): rename function
verify_refname_available(): rename function

Rename is_refname_available() to verify_refname_available() and change
its return value from 1 for success to 0 for success, to be consistent
with our error-handling convention. In a moment it will also get a
"struct strbuf *err" parameter.

Signed-off-by: Michael Haggerty <c24c152f145c57791e809b126db9e61bd1c1782d@alum.mit.edu>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git