upload-pack: add a trigger for post-upload-pack hook

upload-pack: add a trigger for post-upload-pack hook

After upload-pack successfully finishes its operation, post-upload-pack
hook can be called for logging purposes.

The hook is passed various pieces of information, one per line, from its
standard input.  Currently the following items can be fed to the hook, but
more types of information may be added in the future:

    want SHA-1::
        40-byte hexadecimal object name the client asked to include in the
        resulting pack.  Can occur one or more times in the input.

    have SHA-1::
        40-byte hexadecimal object name the client asked to exclude from
        the resulting pack, claiming to have them already.  Can occur zero
        or more times in the input.

    time float::
        Number of seconds spent for creating the packfile.

    size decimal::
        Size of the resulting packfile in bytes.

Signed-off-by: Junio C Hamano <gitster@pobox.com>