templater: fix if() to not evaluate False as bool('False')

Before, False was True. This patch fixes the issue by processing True/False
transparently. The other values (including integer 0) are tested as strings
for backward compatibility, which means "if(latesttagdistance)" never be False.

Should we change the behavior of "if(0)" as well?

mercurial/templater.py

@@ -289,6 +289,15 b' def evalfuncarg(context, mapping, arg):'
         thing = stringify(thing)
     return thing
 
+def evalboolean(context, mapping, arg):
+    func, data = arg
+    thing = func(context, mapping, data)
+    if isinstance(thing, bool):
+        return thing
+    # other objects are evaluated as strings, which means 0 is True, but
+    # empty dict/list should be False as they are expected to be ''
+    return bool(stringify(thing))
+
 def evalinteger(context, mapping, arg, err):
     v = evalfuncarg(context, mapping, arg)
     try:
@@ -560,7 +569,7 b' def if_(context, mapping, args):'
         # i18n: "if" is a keyword
         raise error.ParseError(_("if expects two or three arguments"))
 
-    test = evalstring(context, mapping, args[0])
+    test = evalboolean(context, mapping, args[0])
     if test:
         yield args[1][0](context, mapping, args[1][1])
     elif len(args) == 3:

tests/test-branches.t

@@ -516,6 +516,8 b' template output:'
    }
   ]
 
+  $ hg branches --closed -T '{if(closed, "{branch}\n")}'
+  c
 
 Tests of revision branch name caching