rev-list: support NUL-delimited --missing option

rev-list: support NUL-delimited --missing option

The `--missing={print,print-info}` option for git-rev-list(1) prints
missing objects found while performing the object walk in the form:

        $ git rev-list --missing=print-info <rev>
        ?<oid> [SP <token>=<value>]... LF

Add support for printing missing objects in a NUL-delimited format when
the `-z` option is enabled.

        $ git rev-list -z --missing=print-info <rev>
        <oid> NUL missing=yes NUL [<token>=<value> NUL]...

In this mode, values containing special characters or spaces are printed
as-is without being escaped or quoted. Instead of prefixing the missing
OID with '?', a separate `missing=yes` token/value pair is appended.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>