diff-format.txt: Combined diff format documentation supplement

diff-format.txt: Combined diff format documentation supplement

Update example combined diff format to the current version
$ git diff-tree -p -c fec9ebf16c948bcb4a8b88d0173ee63584bcde76
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>