diff-format.txt: correct misleading wording

Near the end of the "Raw output format" section, an example shows the
output of 'git diff-files' for a tracked file modified on disk but not
yet added to the index. However the wording is:

    <sha1> is shown as all 0's if a file is new on the filesystem
    and it is out of sync with the index.

which is confusing since it can be understood to mean that 'file' is a
new, yet untracked file, in which case 'git diff-files' does not care
about it at all.

When this example was introduced all the way back in c64b9b8
(Reference documentation for the core git commands., 2005-05-05), 'old'
and 'new' referred to the two entities being compared, depending on the
command being used (diff-index, diff-tree or diff-files - which at the
time were diff-cache, diff-tree and show-diff). The wording used at the
time was:

    <new-sha1> is shown as all 0's if new is a file on the
    filesystem and it is out of sync with the cache.

This section was reworked in 81e50ea ([PATCH] The diff-raw
format updates., 2005-05-21) and the mention of the meaning of 'new' and
'old' was removed. Then in f73ae1f (Some typos and light editing of
various manpages, 2005-10-05), the wording was changed to what it is
now.

In addition, in b6d8f30 ([PATCH] diff-raw format update take #2.,
2005-05-23), the section was further reworked and did not use '<sha1>'
anymore, making the example the sole user of this token.

Rework the introductory sentence of the example to instead refer to
'sha1 for "dst"', which is what the text description above it uses, and
fix the wording so that we do not mention a "new file".

While at it, also tweak the wording used in the description of the raw
format to explicitely state that all 0's are used for the destination
hash if the working tree is out of sync with the index, instead of the
more vague "look at worktree".

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-format.txt

@@ -43,7 +43,7 @@ That is, from the left to the right:
 . a space.
 . sha1 for "src"; 0\{40\} if creation or unmerged.
 . a space.
-. sha1 for "dst"; 0\{40\} if deletion, unmerged or "look at work tree".
+. sha1 for "dst"; 0\{40\} if deletion, unmerged or "work tree out of sync with the index".
 . a space.
 . status, followed by optional "score" number.
 . a tab or a NUL when `-z` option is used.
@@ -69,8 +69,8 @@ percentage of similarity between the source and target of the move or
 copy).  Status letter M may be followed by a score (denoting the
 percentage of dissimilarity) for file rewrites.
 
-<sha1> is shown as all 0's if a file is new on the filesystem
-and it is out of sync with the index.
+The sha1 for "dst" is shown as all 0's if a file on the filesystem
+is out of sync with the index.
 
 Example: