templater: introduce unified filter syntax for unary functions

"filter(expr)" is allowed already. This patch adds the opposite, "expr|func".

mercurial/templater.py

@@ -252,6 +252,9 b' def buildfilter(exp, context):'
     if n in context._filters:
         filt = context._filters[n]
         return (runfilter, (func, data, filt))
+    if n in funcs:
+        f = funcs[n]
+        return (f, [(func, data)])
     raise error.ParseError(_("unknown function '%s'") % n)
 
 def runfilter(context, mapping, data):

tests/test-command-template.t

@@ -3117,6 +3117,8 b' Test shortest(node) function:'
   e777603221
   bcc7ff960b
   f7769ec2ab
+  $ hg log --template '{node|shortest}\n' -l1
+  e777
 
 Test pad function