allow nested backquotes, and allow blocks within backquotes.

Reference: /n/atom/patch/applied2013/mkbqnest
Date: Sat Sep  7 18:25:05 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/mk/lex.c	Sat Sep  7 18:24:34 2013
+++ /sys/src/cmd/mk/lex.c	Sat Sep  7 18:24:34 2013
@@ -67,7 +67,7 @@
 static int
 bquote(Biobuf *bp, Bufblock *buf)
 {
-	int c, line, term;
+	int c, line, term, depth;
 	int start;
 
 	line = mkinline;
@@ -79,10 +79,13 @@
 			;
 	} else
 		term = '`';		/* sh style */
+	depth = 1;
 
 	start = buf->current-buf->start;
 	for(;c > 0; c = nextrune(bp, 0)){
-		if(c == term){
+		if(term == '}' && c == '{')
+			depth++;
+		if(c == term && --depth == 0){
 			insert(buf, '\n');
 			insert(buf,0);
 			buf->current = buf->start+start;