builtin/repo: add keyvalue and nul format for structure stats

All repository structure stats are outputted in a human-friendly table
form. This format is not suitable for machine parsing. Add a --format
option that supports three output modes: `table`, `keyvalue`, and `nul`.
The `table` mode is the default format and prints the same table output
as before.

With the `keyvalue` mode, each line of output contains a key-value pair
of a repository stat. The '=' character is used to delimit between keys
and values. The `nul` mode is similar to `keyvalue`, but key-values are
delimited by a NUL character instead of a newline. Also, instead of a
'=' character to delimit between keys and values, a newline character is
used. This allows stat values to support special characters without
having to cquote them. These two new modes provides output that is more
machine-friendly.

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

Documentation/git-repo.adoc

@@ -9,7 +9,7 @@ SYNOPSIS
 --------
 [synopsis]
 git repo info [--format=(keyvalue|nul)] [-z] [<key>...]
-git repo structure
+git repo structure [--format=(table|keyvalue|nul)]
 
 DESCRIPTION
 -----------
@@ -44,7 +44,7 @@ supported:
 +
 `-z` is an alias for `--format=nul`.
 
-`structure`::
+`structure [--format=(table|keyvalue|nul)]`::
 	Retrieve statistics about the current repository structure. The
 	following kinds of information are reported:
 +
@@ -52,7 +52,26 @@ supported:
 * Reachable object counts categorized by type
 
 +
-The table output format may change and is not intended for machine parsing.
+The output format can be chosen through the flag `--format`. Three formats are
+supported:
++
+`table`:::
+	Outputs repository stats in a human-friendly table. This format may
+	change and is not intended for machine parsing. This is the default
+	format.
+
+`keyvalue`:::
+	Each line of output contains a key-value pair for a repository stat.
+	The '=' character is used to delimit between the key and the value.
+	Values containing "unusual" characters are quoted as explained for the
+	configuration variable `core.quotePath` (see linkgit:git-config[1]).
+
+`nul`:::
+	Similar to `keyvalue`, but uses a NUL character to delimit between
+	key-value pairs instead of a newline. Also uses a newline character as
+	the delimiter between the key and value instead of '='. Unlike the
+	`keyvalue` format, values containing "unusual" characters are never
+	quoted.
 
 INFO KEYS
 ---------