ref-filter: add support for %(contents:size)

It's useful and efficient to be able to get the size of the
contents directly without having to pipe through `wc -c`.

Also the result of the following:

`git for-each-ref --format='%(contents)' refs/heads/my-branch | wc -c`

is off by one as `git for-each-ref` appends a newline character
after the contents, which can be seen by comparing its output
with the output from `git cat-file`.

As with %(contents), %(contents:size) is silently ignored, if a
ref points to something other than a commit or a tag:

```
$ git update-ref refs/mytrees/first HEAD^{tree}
$ git for-each-ref --format='%(contents)' refs/mytrees/first

$ git for-each-ref --format='%(contents:size)' refs/mytrees/first

```

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6300-for-each-ref.sh

@@ -52,6 +52,25 @@ test_atom() {
 		sanitize_pgp <actual >actual.clean &&
 		test_cmp expected actual.clean
 	"
+	# Automatically test "contents:size" atom after testing "contents"
+	if test "$2" = "contents"
+	then
+		case $(git cat-file -t "$ref") in
+		tag)
+			# We cannot use $3 as it expects sanitize_pgp to run
+			expect=$(git cat-file tag $ref | tail -n +6 | wc -c) ;;
+		tree | blob)
+			expect='' ;;
+		commit)
+			expect=$(printf '%s' "$3" | wc -c) ;;
+		esac
+		# Leave $expect unquoted to lose possible leading whitespaces
+		echo $expect >expected
+		test_expect_${4:-sucess} $PREREQ "basic atom: $1 contents:size" '
+			git for-each-ref --format="%(contents:size)" "$ref" >actual &&
+			test_cmp expect actual
+		'
+	fi
 }
 
 hexlen=$(test_oid hexsz)