Update sample pre-commit hook to use "diff --check"

Now "diff --check" can detect not just whitespace errors but also notices
leftover conflict marker lines, we can use it in the sample pre-commit
hook script.

These days the object layer knows about the empty tree object without
actually having one in the repository, so we can run the test even for the
initial commit.

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

templates/hooks--pre-commit.sample

@@ -7,64 +7,12 @@
 #
 # To enable this hook, rename this file to "pre-commit".
 
-# This is slightly modified from Andrew Morton's Perfect Patch.
-# Lines you introduce should not have trailing whitespace.
-# Also check for an indentation that has SP before a TAB.
-
 if git-rev-parse --verify HEAD 2>/dev/null
 then
-	git-diff-index -p -M --cached HEAD --
+	against=HEAD
 else
-	# NEEDSWORK: we should produce a diff with an empty tree here
-	# if we want to do the same verification for the initial import.
-	:
-fi |
-perl -e '
-    my $found_bad = 0;
-    my $filename;
-    my $reported_filename = "";
-    my $lineno;
-    sub bad_line {
-	my ($why, $line) = @_;
-	if (!$found_bad) {
-	    print STDERR "*\n";
-	    print STDERR "* You have some suspicious patch lines:\n";
-	    print STDERR "*\n";
-	    $found_bad = 1;
-	}
-	if ($reported_filename ne $filename) {
-	    print STDERR "* In $filename\n";
-	    $reported_filename = $filename;
-	}
-	print STDERR "* $why (line $lineno)\n";
-	print STDERR "$filename:$lineno:$line\n";
-    }
-    while (<>) {
-	if (m|^diff --git a/(.*) b/\1$|) {
-	    $filename = $1;
-	    next;
-	}
-	if (/^@@ -\S+ \+(\d+)/) {
-	    $lineno = $1 - 1;
-	    next;
-	}
-	if (/^ /) {
-	    $lineno++;
-	    next;
-	}
-	if (s/^\+//) {
-	    $lineno++;
-	    chomp;
-	    if (/\s$/) {
-		bad_line("trailing whitespace", $_);
-	    }
-	    if (/^\s* \t/) {
-		bad_line("indent SP followed by a TAB", $_);
-	    }
-	    if (/^([<>])\1{6} |^={7}$/) {
-		bad_line("unresolved merge conflict", $_);
-	    }
-	}
-    }
-    exit($found_bad);
-'
+	# Initial commit: diff against an empty tree object
+	against=4b825dc642cb6eb9a060e54bf8d69288fbee4904
+fi
+
+exec git diff-index --check --cached $against --