grep: print context hunk marks between files

Print a hunk mark before matches from a new file are shown, in addition
to the current behaviour of printing them if lines have been skipped.

The result is easier to read, as (presumably unrelated) matches from
different files are separated by a hunk mark.  GNU grep does the same.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7002-grep.sh

@@ -155,6 +155,28 @@ test_expect_success 'grep -e A --and --not -e B' '
 	test_cmp expected actual
 '
 
+cat >expected <<EOF
+y:y yy
+--
+z:zzz
+EOF
+
+# Create 1024 file names that sort between "y" and "z" to make sure
+# the two files are handled by different calls to an external grep.
+# This depends on MAXARGS in builtin-grep.c being 1024 or less.
+c32="0 1 2 3 4 5 6 7 8 9 a b c d e f g h i j k l m n o p q r s t u v"
+test_expect_success 'grep -C1, hunk mark between files' '
+	for a in $c32; do for b in $c32; do : >y-$a$b; done; done &&
+	git add y-?? &&
+	git grep -C1 "^[yz]" >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'grep -C1 --no-ext-grep, hunk mark between files' '
+	git grep -C1 --no-ext-grep "^[yz]" >actual &&
+	test_cmp expected actual
+'
+
 test_expect_success 'log grep setup' '
 	echo a >>file &&
 	test_tick &&