Fix documentation dependency generation.

Documentation/Makefile spent a lot of time to generate include
dependencies, which was quite noticeable especially during "make clean".

Rewrite it to generate just a single dependency file.

Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/build-docdep.perl

@@ -0,0 +1,28 @@
+#!/usr/bin/perl
+
+my %include = ();
+
+for my $text (<git-*.txt>) {
+    open I, '<', $text || die "cannot read: $text";
+    (my $base = $text) =~ s/\.txt$//;
+    while (<I>) {
+	if (/^include::/) {
+	    chomp;
+	    s/^include::\s*//;
+	    s/\[\]//;
+	    $include{$base}{$_} = 1;
+	}
+    }
+    close I;
+}
+
+# Do we care about chained includes???
+
+while (my ($base, $included) = each %include) {
+    my ($suffix) = '1';
+    if ($base eq 'git') {
+	$suffix = '7'; # yuck...
+    }
+    print "$base.html $base.$suffix : ", join(" ", keys %$included), "\n";
+}
+