[MERGE] Add mechanism for pre_change_branch_tip hook to cleanly reject a change.

[Andrew Bennetts]

Hi,

This patch adds a TipChangeRejected error for pre_change_branch_tip hooks to
raise.  The message given to TipChangeRejected will be reported directly to the
user, even over the smart server.  This is important for a hook that wants to
explain to the user *why* a change is rejected (e.g. "foo.xml is not valid XML:
syntax error on line 1234").

This patch is relative to my last patch for a smart server hooks integration
test, but doesn't really depend on it.

-Andrew.

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