Add post-merge hook, related documentation, and tests.

The post-merge hook enables one to hook in for `git pull` operations in order
to check and/or change attributes of a work tree from the hook.  As an example,
it can be used in combination with a pre-commit hook to save/restore file
ownership and permissions data (or file ACLs) within the repository and
transparently update the working tree after a `git pull` operation.

Signed-off-by: Josh England <jjengla@sandia.gov>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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