diff-format.txt: Combined diff format documentation supplement

Update example combined diff format to the current version
$ git diff-tree -p -c fec9ebf
and provide complete first chunk in example.

Document combined diff format headers: how "diff header" look like,
which of "extended diff headers" are used with combined diff and how
they look like, differences in two-line from-file/to-file header from
non-combined diff format, chunk header format.

It should be noted that combined diff format was designed for quick
_content_ inspection and renames would work correctly to pick which
blobs from each tree to compare but otherwise not reflected in the
output (the pathnames are not shown).

[jc: with minimum copyediting]

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/diff-format.txt

@@ -156,31 +156,91 @@ to produce 'combined diff', which looks like this:
 
 ------------
 diff --combined describe.c
-@@@ +98,7 @@@
-   return (a_date > b_date) ? -1 : (a_date == b_date) ? 0 : 1;
+index fabadb8,cc95eb0..4866510
+--- a/describe.c
++++ b/describe.c
+@@@ -98,20 -98,12 +98,20 @@@
+  	return (a_date > b_date) ? -1 : (a_date == b_date) ? 0 : 1;
   }
-
+  
 - static void describe(char *arg)
  -static void describe(struct commit *cmit, int last_one)
 ++static void describe(char *arg, int last_one)
   {
- +     unsigned char sha1[20];
- +     struct commit *cmit;
+ +	unsigned char sha1[20];
+ +	struct commit *cmit;
+  	struct commit_list *list;
+  	static int initialized = 0;
+  	struct commit_name *n;
+
+ +	if (get_sha1(arg, sha1) < 0)
+ +		usage(describe_usage);
+ +	cmit = lookup_commit_reference(sha1);
+ +	if (!cmit)
+ +		usage(describe_usage);
+ +
+  	if (!initialized) {
+  		initialized = 1;
+  		for_each_ref(get_name);
 ------------
 
+1.   It is preceded with a "git diff" header, that looks like
+     this (when '-c' option is used):
+
+       diff --combined file
++
+or like this (when '--cc' option is used):
+
+       diff --c file
+
+2.   It is followed by one or more extended header lines
+     (this example shows a merge with two parents):
+
+       index <hash>,<hash>..<hash>
+       mode <mode>,<mode>..<mode>
+       new file mode <mode>
+       deleted file mode <mode>,<mode>
++
+The `mode <mode>,<mode>..<mode>` line appears only if at least one of
+the <mode> is diferent from the rest. Extended headers with
+information about detected contents movement (renames and
+copying detection) are designed to work with diff of two
+<tree-ish> and are not used by combined diff format.
+
+3.   It is followed by two-line from-file/to-file header
+
+       --- a/file
+       +++ b/file
++
+Contrary to two-line header for traditional 'unified' diff
+format, and similar to filenames in ordinary "diff header",
+/dev/null is not used for creation or deletion.
+
+4.   Chunk header format is modified to prevent people from
+     accidentally feeding it to `patch -p1`. Combined diff format
+     was created for review of merge commit changes, and was not
+     meant for apply. The change is similar to the change in the
+     extended 'index' header:
+
+       @@@ <from-file-range> <from-file-range> <to-file-range> @@@
++
+There are (number of parents + 1) `@` characters in the chunk
+header for combined diff format.
+
 Unlike the traditional 'unified' diff format, which shows two
 files A and B with a single column that has `-` (minus --
 appears in A but removed in B), `+` (plus -- missing in A but
-added to B), or ` ` (space -- unchanged) prefix, this format
+added to B), or `" "` (space -- unchanged) prefix, this format
 compares two or more files file1, file2,... with one file X, and
 shows how X differs from each of fileN.  One column for each of
 fileN is prepended to the output line to note how X's line is
 different from it.
 
 A `-` character in the column N means that the line appears in
-fileN but it does not appear in the last file.  A `+` character
+fileN but it does not appear in the result.  A `+` character
 in the column N means that the line appears in the last file,
-and fileN does not have that line.
+and fileN does not have that line (in other words, the line was
+added, from the point of view of that parent).
 
 In the above example output, the function signature was changed
 from both files (hence two `-` removals from both file1 and