diff-format.adoc: remove mention of diff-tree specific output

diff-format.adoc: remove mention of diff-tree specific output

In the "Raw output format" section, we start by mentioning that 'git
diff-tree' prints the hashes of what is being compared. This is only
true in --stdin mode, and is already mentioned in the description of
'--stdin' in git-diff-tree.adoc. Remove this sentence such that we only
focus on the common output between diff-tree, diff-index, diff-files and

Signed-off-by: Junio C Hamano <gitster@pobox.com>