diff --check: do not get confused by new blank lines in the middle

The code remembered that the last diff output it saw was an empty line,
and tried to reset that state whenever it sees a context line, a non-blank
new line, or a new hunk.  However, this codepath asks the underlying diff
engine to feed diff without any context, and the "just saw an empty line"
state was not reset if you added a new blank line in the last hunk of your
patch, even if it is not the last line of the file.

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

diff.c

@@ -1627,6 +1627,7 @@ static void builtin_checkdiff(const char *name_a, const char *name_b,
 		xdemitcb_t ecb;
 
 		memset(&xecfg, 0, sizeof(xecfg));
+		xecfg.ctxlen = 1; /* at least one context line */
 		xpp.flags = XDF_NEED_MINIMAL;
 		ecb.outf = xdiff_outf;
 		ecb.priv = &data;

t/t4015-diff-whitespace.sh

@@ -341,4 +341,15 @@ test_expect_success 'checkdiff detects trailing blank lines' '
 	git diff --check | grep "ends with blank"
 '
 
+test_expect_success 'checkdiff allows new blank lines' '
+	git checkout x &&
+	mv x y &&
+	(
+		echo "/* This is new */" &&
+		echo "" &&
+		cat y
+	) >x &&
+	git diff --check
+'
+
 test_done