tools/post-receive.hook: don't update the previously pushed commit

tools/post-receive.hook: don't update the previously pushed commit

Previously, the post-receive hook would always examine one commit that
had been previously pushed, (when the purpose of the hook is only to
example newly-pushed commits). We fix this by simply dropping the '^'
in the commit-range specification.

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