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

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

The `--boundary` option for git-rev-list(1) prints boundary objects
found while performing the object walk in the form:

        $ git rev-list --boundary <rev>
        -<oid> LF

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

        $ git rev-list -z --boundary <rev>
        <oid> NUL boundary=yes NUL

In this mode, instead of prefixing the boundary OID with '-', a separate
`boundary=yes` token/value pair is appended.

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