merge(third_party/git): Merge squashed git subtree at v2.23.0

Merge commit '1b593e1ea4' as 'third_party/git'

third_party/git/Documentation/build-docdep.perl

@@ -0,0 +1,46 @@
+#!/usr/bin/perl
+
+my %include = ();
+my %included = ();
+
+for my $text (<*.txt>) {
+    open I, '<', $text || die "cannot read: $text";
+    while (<I>) {
+	if (/^include::/) {
+	    chomp;
+	    s/^include::\s*//;
+	    s/\[\]//;
+	    $include{$text}{$_} = 1;
+	    $included{$_} = 1;
+	}
+    }
+    close I;
+}
+
+# Do we care about chained includes???
+my $changed = 1;
+while ($changed) {
+    $changed = 0;
+    while (my ($text, $included) = each %include) {
+	for my $i (keys %$included) {
+	    # $text has include::$i; if $i includes $j
+	    # $text indirectly includes $j.
+	    if (exists $include{$i}) {
+		for my $j (keys %{$include{$i}}) {
+		    if (!exists $include{$text}{$j}) {
+			$include{$text}{$j} = 1;
+			$included{$j} = 1;
+			$changed = 1;
+		    }
+		}
+	    }
+	}
+    }
+}
+
+while (my ($text, $included) = each %include) {
+    if (! exists $included{$text} &&
+	(my $base = $text) =~ s/\.txt$//) {
+	print "$base.html $base.xml : ", join(" ", keys %$included), "\n";
+    }
+}