tests: fix and add lint for non-portable head -c N

The "head -c BYTES" option is non-portable (not in POSIX[1]). Change
such invocations to use the test_copy_bytes wrapper added in
48860819e8 ("t9300: factor out portable "head -c" replacement",
2016-06-30).

This fixes a test added in 9d2e330b17 ("ewah_read_mmap: bounds-check
mmap reads", 2018-06-14), which has been breaking
t5310-pack-bitmaps.sh on OpenBSD since 2.18.0. The OpenBSD ports
already have a similar workaround after their upgrade to 2.18.0[2].

I have not tested this on IRIX, but according to 4de0bbd898 ("t9300:
use perl "head -c" clone in place of "dd bs=1 count=16000" kluge",
2010-12-13) this invocation would have broken things there too.

Also, change a valgrind-specific codepath in test-lib.sh to use this
wrapper. Given where valgrind runs I don't think this would ever
become a portability issue in practice, but it's easier to just use
the wrapper than introduce some exception for the "make test-lint"
check being added here.

1. http://pubs.opengroup.org/onlinepubs/9699919799/utilities/head.html
2. 08d5d82eae (diff-f7d3c4fabeed1691620d608f1534f5e5)

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/check-non-portable-shell.pl

@@ -41,6 +41,7 @@ while (<>) {
 	/^\s*[^#]\s*which\s/ and err 'which is not portable (use type)';
 	/\btest\s+[^=]*==/ and err '"test a == b" is not portable (use =)';
 	/\bwc -l.*"\s*=/ and err '`"$(wc -l)"` is not portable (use test_line_count)';
+	/\bhead\s+-c\b/ and err 'head -c is not portable (use test_copy_bytes BYTES <file >out)';
 	/\bexport\s+[A-Za-z0-9_]*=/ and err '"export FOO=bar" is not portable (use FOO=bar && export FOO)';
 	/^\s*([A-Z0-9_]+=(\w+|(["']).*?\3)\s+)+(\w+)/ and exists($func{$4}) and
 		err '"FOO=bar shell_func" assignment extends beyond "shell_func"';

t/t5310-pack-bitmaps.sh

@@ -335,7 +335,7 @@ test_expect_success 'truncated bitmap fails gracefully' '
 	git rev-list --use-bitmap-index --count --all >expect &&
 	bitmap=$(ls .git/objects/pack/*.bitmap) &&
 	test_when_finished "rm -f $bitmap" &&
-	head -c 512 <$bitmap >$bitmap.tmp &&
+	test_copy_bytes 512 <$bitmap >$bitmap.tmp &&
 	mv -f $bitmap.tmp $bitmap &&
 	git rev-list --use-bitmap-index --count --all >actual 2>stderr &&
 	test_cmp expect actual &&

t/test-lib.sh

@@ -867,7 +867,7 @@ then
 		# handle only executables, unless they are shell libraries that
 		# need to be in the exec-path.
 		test -x "$1" ||
-		test "# " = "$(head -c 2 <"$1")" ||
+		test "# " = "$(test_copy_bytes 2 <"$1")" ||
 		return;
 
 		base=$(basename "$1")
@@ -882,7 +882,7 @@ then
 		# do not override scripts
 		if test -x "$symlink_target" &&
 		    test ! -d "$symlink_target" &&
-		    test "#!" != "$(head -c 2 < "$symlink_target")"
+		    test "#!" != "$(test_copy_bytes 2 <"$symlink_target")"
 		then
 			symlink_target=../valgrind.sh
 		fi