[FIX] qweb: correctly handle top level t-call and body

Whenever a top level t-call was made with some non empty body, Owl
complained that a template should not have more than one root node.

The reason was that the compilation context for the body of the t-call
directive was the same as the root compilation context, and it already
had a parentNode set.

To fix the issue, this commit simply use the subContext method to create
a different compilation context, which actually makes sense, because the
body of a t-call is really a different situation.  Also, as a bonus, it
slightly improves the code for the t-call directive.

closes #760

src/qweb/base_directives.ts

@@ -267,16 +267,13 @@ QWeb.addDirective({
       for (let attr of ["t-if", "t-else", "t-elif", "t-call"]) {
         nodeCopy.removeAttribute(attr);
       }
-      const parentNode = ctx.parentNode;
-      ctx.parentNode = "__0";
       // this local scope is intended to trap c__0
       ctx.addLine(`{`);
       ctx.indent();
       ctx.addLine("let c__0 = [];");
-      qweb._compileNode(nodeCopy, ctx);
+      qweb._compileNode(nodeCopy, ctx.subContext("parentNode", "__0"));
       ctx.rootContext.shouldDefineUtils = true;
       ctx.addLine("scope[utils.zero] = c__0;");
-      ctx.parentNode = parentNode;
       ctx.dedent();
       ctx.addLine(`}`);
     }

tests/qweb/__snapshots__/qweb.test.ts.snap

@@ -1798,6 +1798,35 @@ exports[`t-call (template calling t-call allowed on a non t node 1`] = `
 }"
 `;
 
+exports[`t-call (template calling t-call with body content as root of a template 1`] = `
+"function anonymous(context, extra
+) {
+    // Template name: \\"main\\"
+    let utils = this.constructor.utils;
+    let scope = Object.create(context);
+    let result;
+    let h = this.h;
+    let _origScope3 = scope;
+    scope = Object.create(scope);
+    scope.__access_mode__ = 'ro';
+    {
+        {
+            let c__0 = [];
+            let c4 = [], p4 = {key:4};
+            let vn4 = h('p', p4, c4);
+            c__0.push(vn4);
+            c4.push({text: \`antony\`});
+            scope[utils.zero] = c__0;
+        }
+        result = []
+        this.constructor.subTemplates['1'].call(this, scope, Object.assign({}, extra, {parentNode: result, parent: utils.getComponent(context), key: '__5__'}));
+        result = result[0]
+    }
+    scope = _origScope3;
+    return result;
+}"
+`;
+
 exports[`t-call (template calling t-call with t-if 1`] = `
 "function anonymous(context, extra
 ) {

tests/qweb/qweb.test.ts

@@ -1096,6 +1096,13 @@ describe("t-call (template calling", () => {
     expect(renderToString(qweb, "main")).toBe(expected);
   });
 
+  test("t-call with body content as root of a template", () => {
+    qweb.addTemplate("antony", `<foo><t t-raw="0"/></foo>`);
+    qweb.addTemplate("main", `<t><t t-call="antony"><p>antony</p></t></t>`);
+    const expected = "<foo><p>antony</p></foo>";
+    expect(renderToString(qweb, "main")).toBe(expected);
+  });
+
   test("dynamic t-call", () => {
     qweb.addTemplate("foo", `<foo><t t-esc="val"/></foo>`);
     qweb.addTemplate("bar", `<bar><t t-esc="val"/></bar>`);