word-diff: ignore '\ No newline at eof' marker

The word-diff logic accumulates + and - lines until another line type
appears (normally [ @\]), at which point it generates the word diff.
This is usually correct, but it breaks when the preimage does not have
a newline at EOF:

  $ printf "%s" "a a a" >a
  $ printf "%s\n" "a ab a" >b
  $ git diff --no-index --word-diff a b
  diff --git 1/a 2/b
  index 9f68e94..6a7c02f 100644
  --- 1/a
  +++ 2/b
  @@ -1 +1 @@
  [-a a a-]
   No newline at end of file
  {+a ab a+}

Because of the order of the lines in a unified diff

  @@ -1 +1 @@
  -a a a
  \ No newline at end of file
  +a ab a

the '\' line flushed the buffers, and the - and + lines were never
matched with each other.

A proper fix would defer such markers until the end of the hunk.
However, word-diff is inherently whitespace-ignoring, so as a cheap
fix simply ignore the marker (and hide it from the output).

We use a prefix match for '\ ' to parallel the logic in
apply.c:parse_fragment().  We currently do not localize this string
(just accept other variants of it in git-apply), but this should be
future-proof.

Noticed-by: Ivan Shirokoff <shirokoff@yandex-team.ru>
Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4034-diff-words.sh

@@ -333,4 +333,18 @@ test_expect_success 'word-diff with diff.sbe' '
 	word_diff --word-diff=plain
 '
 
+test_expect_success 'word-diff with no newline at EOF' '
+	cat >expect <<-\EOF &&
+	diff --git a/pre b/post
+	index 7bf316e..3dd0303 100644
+	--- a/pre
+	+++ b/post
+	@@ -1 +1 @@
+	a a [-a-]{+ab+} a a
+	EOF
+	printf "%s" "a a a a a" >pre &&
+	printf "%s" "a a ab a a" >post &&
+	word_diff --word-diff=plain
+'
+
 test_done