rev-list: support delimiting objects with NUL bytes

When walking objects, git-rev-list(1) prints each object entry on a
separate line. Some options, such as `--objects`, may print additional
information about tree and blob object on the same line in the form:

        $ git rev-list --objects <rev>
        <tree/blob oid> SP [<path>] LF

Note that in this form the SP is appended regardless of whether the tree
or blob object has path information available. Paths containing a
newline are also truncated at the newline.

Introduce the `-z` option for git-rev-list(1) which reformats the output
to use NUL-delimiters between objects and associated info in the
following form:

        $ git rev-list -z --objects <rev>
        <oid> NUL [path=<path> NUL]

In this form, the start of each record is signaled by an OID entry that
is all hexidecimal and does not contain any '='. Additional path info
from `--objects` is appended to the record as a token/value pair
`path=<path>` as-is without any truncation.

For now, the `--objects` flag is the only options that can be used in
combination with `-z`. In a subsequent commit, NUL-delimited support for
other options is added. Other options that do not make sense when used
in combination with `-z` are rejected.

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

Documentation/rev-list-options.adoc

@@ -361,6 +361,27 @@ ifdef::git-rev-list[]
 --progress=<header>::
 	Show progress reports on stderr as objects are considered. The
 	`<header>` text will be printed with each progress update.
+
+-z::
+	Instead of being newline-delimited, each outputted object and its
+	accompanying metadata is delimited using NUL bytes. Output is printed
+	in the following form:
++
+-----------------------------------------------------------------------
+<OID> NUL [<token>=<value> NUL]...
+-----------------------------------------------------------------------
++
+Additional object metadata, such as object paths, is printed using the
+`<token>=<value>` form. Token values are printed as-is without any
+encoding/truncation. An OID entry never contains a '=' character and thus
+is used to signal the start of a new object record. Examples:
++
+-----------------------------------------------------------------------
+<OID> NUL
+<OID> NUL path=<path> NUL
+-----------------------------------------------------------------------
++
+This mode is only compatible with the `--objects` output option.
 endif::git-rev-list[]
 
 History Simplification