Documentation/git-diff-tree.txt: fix formatting

Documentation/git-diff-tree.txt: fix formatting

Asciidoctor versions v1.5.7 or later print the following warning while
building the documentation:

      ASCIIDOC git-diff-tree.xml
  asciidoctor: WARNING: diff-format.txt: line 2: unterminated listing block

This highlights an issue (even with older Asciidoctor versions) where
the "Raw output format" header is not rendered as a header, and the
rest of the document is rendered in monospace.  This is not caused by
'diff-format.txt' in itself, but rather by 'git-diff-tree.txt'
including 'pretty-formats.txt' and 'diff-format.txt' on subsequent
lines, while the former happens to end with monospace-formatted
example commands.

Fix this by inserting an empty line between the two include::
directives.

The page rendered with AsciiDoc doesn't have this formatting issue.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>