diff-format.adoc: 'git diff-files' prints two lines for unmerged files

diff-format.adoc: 'git diff-files' prints two lines for unmerged files

Since 10637b84d9 (diff-files: -1/-2/-3 to diff against unmerged stage.,
2005-11-29), for unmerged entries 'git diff-files' print both an
"unmerged" line ('U'), as well as an "in-place edit" line ('M')
comparing stage 2 (by default) with the working tree. The "Raw output
format" documentation however mentions that all commands print a single
line per changed file. Adjust diff-format.adoc to also mention this
special case, for completeness.

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