post-receive: Fix error message when pushing to an unmapped branch

post-receive: Fix error message when pushing to an unmapped branch

Previously, if STATE_MAP contained only refs/heads/master and a user
pushed to a branch such as refs/heads/stable, the hook emitted a
misleading error message:

	E: no mapping for refname refs/heads/master

Fix this by correctly printing the reference being looked for, rather
than the reference most recently found in STATE_MAP. Also, reword the
message slightly to point the user to STATE_MAP and to use an actual
word "branch" instead of the non-word "refname".

Signed-off-by: Carl Worth <cworth@cworth.org>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>