detect pagespecs that require content dependencies

When adding a contentless dependency, the pagespec also needs to be one
that does not look at any page content information.

As a first approximation of that, only allow glob-based pagespecs in
contentless dependencies. While there are probably a few other types of
pagespecs that can match contentless, this will work for most of them.

IkiWiki.pm

@@ -1774,7 +1774,8 @@ sub add_depends ($$;@) {
 	my $deptype=$DEPEND_CONTENT | $DEPEND_EXISTS;
 	if (@_) {
 		my %params=@_;
-		if (defined $params{content} && $params{content} == 0) {
+		if (defined $params{content} && $params{content} == 0 &&
+		    pagespec_contentless($pagespec)) {
 			$deptype=$deptype & ~$DEPEND_CONTENT;
 		}
 	}
@@ -1974,6 +1975,20 @@ sub pagespec_valid ($) {
 	return ! $@;
 }
 
+sub pagespec_contentless ($) {
+	my $spec=shift;
+
+	while ($spec=~m{
+		(\w+)\([^\)]*\) # only match pagespec functions
+	}igx) {
+		# only glob and internal can be matched contentless
+		# (first approximation)
+		return 0 if $1 ne "glob" && $1 ne "internal";
+	}
+
+	return 1;
+}
+
 sub glob2re ($) {
 	my $re=quotemeta(shift);
 	$re=~s/\\\*/.*/g;