diff: print line prefix for --name-only output

If you run "git log --graph --name-only", the pathnames are
not indented to go along with their matching commits (unlike
all of the other diff formats). We need to output the line
prefix for each item before writing it.

The tests cover both --name-status and --name-only. The
former actually gets this right already, because it builds
on the --raw format functions. It's only --name-only which
uses its own code (and this fix mirrors the code in
diff_flush_raw()).

Note that the tests don't follow our usual style of setting
up the "expect" output inside the test block. This matches
the surrounding style, but more importantly it is easier to
read: we don't have to worry about embedded single-quotes,
and the leading indentation is more obvious.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

diff.c

@@ -4353,6 +4353,7 @@ static void flush_one_pair(struct diff_filepair *p, struct diff_options *opt)
 		name_a = p->two->path;
 		name_b = NULL;
 		strip_prefix(opt->prefix_length, &name_a, &name_b);
+		fprintf(opt->file, "%s", diff_line_prefix(opt));
 		write_name_quoted(name_a, opt->file, opt->line_termination);
 	}
 }

t/t4202-log.sh

@@ -867,6 +867,54 @@ test_expect_success 'log --graph with diff and stats' '
 	test_i18ncmp expect actual.sanitized
 '
 
+cat >expect <<-\EOF
+* reach
+|
+| A	reach.t
+* Merge branch 'tangle'
+*   Merge branch 'side'
+|\
+| * side-2
+|
+|   A	2
+* Second
+|
+| A	one
+* sixth
+
+  D	a/two
+EOF
+
+test_expect_success 'log --graph with --name-status' '
+	git log --graph --format=%s --name-status tangle..reach >actual &&
+	sanitize_output <actual >actual.sanitized &&
+	test_cmp expect actual.sanitized
+'
+
+cat >expect <<-\EOF
+* reach
+|
+| reach.t
+* Merge branch 'tangle'
+*   Merge branch 'side'
+|\
+| * side-2
+|
+|   2
+* Second
+|
+| one
+* sixth
+
+  a/two
+EOF
+
+test_expect_success 'log --graph with --name-only' '
+	git log --graph --format=%s --name-only tangle..reach >actual &&
+	sanitize_output <actual >actual.sanitized &&
+	test_cmp expect actual.sanitized
+'
+
 test_expect_success 'dotdot is a parent directory' '
 	mkdir -p a/b &&
 	( echo sixth && echo fifth ) >expect &&