doc: clarify the filename encoding in git diff

doc: clarify the filename encoding in git diff

AFAICT parsing the output of `git diff --name-only master...feature`
is the intended way of programmatically getting the list of files
modified
by a feature branch. It is impossible to parse text unless you know what
encoding it is in. The output encoding of diff --name-only and

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