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>

1 files changed, 3 insertions, 0 deletions

diff --git a/Documentation/git-for-each-ref.txt b/Documentation/git-for-each-ref.txt
index b739412..2ea71c5 100644
--- a/Documentation/git-for-each-ref.txt
+++ b/Documentation/git-for-each-ref.txt
@@ -235,6 +235,9 @@ and `date` to extract the named component.
 The message in a commit or a tag object is `contents`, from which
 `contents:<part>` can be used to extract various parts out of:
 
+contents:size::
+	The size in bytes of the commit or tag message.
+
 contents:subject::
 	The first paragraph of the message, which typically is a
 	single line, is taken as the "subject" of the commit or the