t4108: remove git command upstream of pipe

Before, the output of `git diff HEAD` would always be piped to
sanitize_conflicted_diff(). However, since the Git command was upstream
of the pipe, in case the Git command fails, the return code would be
lost. Rewrite into separate statements so that the return code is no
longer lost.

Since only the command `git diff HEAD` was being piped to
sanitize_conflicted_diff(), move the command into the function and rename
it to print_sanitized_conflicted_diff().

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4108-apply-threeway.sh

@@ -4,11 +4,12 @@ test_description='git apply --3way'
 
 . ./test-lib.sh
 
-sanitize_conflicted_diff () {
+print_sanitized_conflicted_diff () {
+	git diff HEAD >diff.raw &&
 	sed -e '
 		/^index /d
 		s/^\(+[<>][<>][<>][<>]*\) .*/\1/
-	'
+	' diff.raw
 }
 
 test_expect_success setup '
@@ -54,14 +55,14 @@ test_expect_success 'apply with --3way' '
 	git checkout master^0 &&
 	test_must_fail git merge --no-commit side &&
 	git ls-files -s >expect.ls &&
-	git diff HEAD | sanitize_conflicted_diff >expect.diff &&
+	print_sanitized_conflicted_diff >expect.diff &&
 
 	# should fail to apply
 	git reset --hard &&
 	git checkout master^0 &&
 	test_must_fail git apply --index --3way P.diff &&
 	git ls-files -s >actual.ls &&
-	git diff HEAD | sanitize_conflicted_diff >actual.diff &&
+	print_sanitized_conflicted_diff >actual.diff &&
 
 	# The result should resemble the corresponding merge
 	test_cmp expect.ls actual.ls &&
@@ -114,7 +115,7 @@ test_expect_success 'apply -3 with add/add conflict setup' '
 	git checkout adder^0 &&
 	test_must_fail git merge --no-commit another &&
 	git ls-files -s >expect.ls &&
-	git diff HEAD | sanitize_conflicted_diff >expect.diff
+	print_sanitized_conflicted_diff >expect.diff
 '
 
 test_expect_success 'apply -3 with add/add conflict' '
@@ -124,7 +125,7 @@ test_expect_success 'apply -3 with add/add conflict' '
 	test_must_fail git apply --index --3way P.diff &&
 	# ... and leave conflicts in the index and in the working tree
 	git ls-files -s >actual.ls &&
-	git diff HEAD | sanitize_conflicted_diff >actual.diff &&
+	print_sanitized_conflicted_diff >actual.diff &&
 
 	# The result should resemble the corresponding merge
 	test_cmp expect.ls actual.ls &&