t4015: avoid git as a pipe input

In t4015 we have a pattern of

    git diff [<options, related to color>] |
        grep -v "index" |
        test_decode_color >actual &&

to produce output that we want to test against. This pattern was introduced
in 86b452e276 (diff.c: add dimming to moved line detection, 2017-06-30)
as then the focus on getting the colors right. However the pattern used
is not best practice as we do care about the exit code of Git. So let's
not have Git as the upstream of a pipe. Piping the output of grep to
some function is fine as we assume grep to be un-flawed in our test suite.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4015-diff-whitespace.sh

@@ -1271,9 +1271,8 @@ test_expect_success 'detect permutations inside moved code -- dimmed_zebra' '
 	test_config color.diff.newMovedDimmed "normal cyan" &&
 	test_config color.diff.oldMovedAlternativeDimmed "normal blue" &&
 	test_config color.diff.newMovedAlternativeDimmed "normal yellow" &&
-	git diff HEAD --no-renames --color-moved=dimmed_zebra --color |
-		grep -v "index" |
-		test_decode_color >actual &&
+	git diff HEAD --no-renames --color-moved=dimmed_zebra --color >actual.raw &&
+	grep -v "index" actual.raw | test_decode_color >actual &&
 	cat <<-\EOF >expected &&
 	<BOLD>diff --git a/lines.txt b/lines.txt<RESET>
 	<BOLD>--- a/lines.txt<RESET>
@@ -1315,9 +1314,8 @@ test_expect_success 'cmd option assumes configured colored-moved' '
 	test_config color.diff.oldMovedAlternativeDimmed "normal blue" &&
 	test_config color.diff.newMovedAlternativeDimmed "normal yellow" &&
 	test_config diff.colorMoved zebra &&
-	git diff HEAD --no-renames --color-moved --color |
-		grep -v "index" |
-		test_decode_color >actual &&
+	git diff HEAD --no-renames --color-moved --color >actual.raw &&
+	grep -v "index" actual.raw | test_decode_color >actual &&
 	cat <<-\EOF >expected &&
 	<BOLD>diff --git a/lines.txt b/lines.txt<RESET>
 	<BOLD>--- a/lines.txt<RESET>
@@ -1395,9 +1393,8 @@ test_expect_success 'move detection ignoring whitespace ' '
 	line 4
 	line 5
 	EOF
-	git diff HEAD --no-renames --color-moved --color |
-		grep -v "index" |
-		test_decode_color >actual &&
+	git diff HEAD --no-renames --color-moved --color >actual.raw &&
+	grep -v "index" actual.raw | test_decode_color >actual &&
 	cat <<-\EOF >expected &&
 	<BOLD>diff --git a/lines.txt b/lines.txt<RESET>
 	<BOLD>--- a/lines.txt<RESET>
@@ -1419,9 +1416,8 @@ test_expect_success 'move detection ignoring whitespace ' '
 	EOF
 	test_cmp expected actual &&
 
-	git diff HEAD --no-renames -w --color-moved --color |
-		grep -v "index" |
-		test_decode_color >actual &&
+	git diff HEAD --no-renames -w --color-moved --color >actual.raw &&
+	grep -v "index" actual.raw | test_decode_color >actual &&
 	cat <<-\EOF >expected &&
 	<BOLD>diff --git a/lines.txt b/lines.txt<RESET>
 	<BOLD>--- a/lines.txt<RESET>
@@ -1459,9 +1455,8 @@ test_expect_success 'move detection ignoring whitespace changes' '
 	line 5
 	EOF
 
-	git diff HEAD --no-renames --color-moved --color |
-		grep -v "index" |
-		test_decode_color >actual &&
+	git diff HEAD --no-renames --color-moved --color >actual.raw &&
+	grep -v "index" actual.raw | test_decode_color >actual &&
 	cat <<-\EOF >expected &&
 	<BOLD>diff --git a/lines.txt b/lines.txt<RESET>
 	<BOLD>--- a/lines.txt<RESET>
@@ -1483,9 +1478,8 @@ test_expect_success 'move detection ignoring whitespace changes' '
 	EOF
 	test_cmp expected actual &&
 
-	git diff HEAD --no-renames -b --color-moved --color |
-		grep -v "index" |
-		test_decode_color >actual &&
+	git diff HEAD --no-renames -b --color-moved --color >actual.raw &&
+	grep -v "index" actual.raw | test_decode_color >actual &&
 	cat <<-\EOF >expected &&
 	<BOLD>diff --git a/lines.txt b/lines.txt<RESET>
 	<BOLD>--- a/lines.txt<RESET>
@@ -1526,9 +1520,8 @@ test_expect_success 'move detection ignoring whitespace at eol' '
 	# avoid cluttering the output with complaints about our eol whitespace
 	test_config core.whitespace -blank-at-eol &&
 
-	git diff HEAD --no-renames --color-moved --color |
-		grep -v "index" |
-		test_decode_color >actual &&
+	git diff HEAD --no-renames --color-moved --color >actual.raw &&
+	grep -v "index" actual.raw | test_decode_color >actual &&
 	cat <<-\EOF >expected &&
 	<BOLD>diff --git a/lines.txt b/lines.txt<RESET>
 	<BOLD>--- a/lines.txt<RESET>
@@ -1550,9 +1543,8 @@ test_expect_success 'move detection ignoring whitespace at eol' '
 	EOF
 	test_cmp expected actual &&
 
-	git diff HEAD --no-renames --ignore-space-at-eol --color-moved --color |
-		grep -v "index" |
-		test_decode_color >actual &&
+	git diff HEAD --no-renames --ignore-space-at-eol --color-moved --color >actual.raw &&
+	grep -v "index" actual.raw | test_decode_color >actual &&
 	cat <<-\EOF >expected &&
 	<BOLD>diff --git a/lines.txt b/lines.txt<RESET>
 	<BOLD>--- a/lines.txt<RESET>
@@ -1597,9 +1589,8 @@ test_expect_success '--color-moved block at end of diff output respects MIN_ALNU
 	irrelevant_line
 	EOF
 
-	git diff HEAD --color-moved=zebra --color --no-renames |
-		grep -v "index" |
-		test_decode_color >actual &&
+	git diff HEAD --color-moved=zebra --color --no-renames >actual.raw &&
+	grep -v "index" actual.raw | test_decode_color >actual &&
 	cat >expected <<-\EOF &&
 	<BOLD>diff --git a/bar b/bar<RESET>
 	<BOLD>--- a/bar<RESET>
@@ -1636,9 +1627,8 @@ test_expect_success '--color-moved respects MIN_ALNUM_COUNT' '
 	nineteen chars 456789
 	EOF
 
-	git diff HEAD --color-moved=zebra --color --no-renames |
-		grep -v "index" |
-		test_decode_color >actual &&
+	git diff HEAD --color-moved=zebra --color --no-renames >actual.raw &&
+	grep -v "index" actual.raw | test_decode_color >actual &&
 	cat >expected <<-\EOF &&
 	<BOLD>diff --git a/bar b/bar<RESET>
 	<BOLD>--- a/bar<RESET>