templater: allow piping generator-type function output to filters

Template functions use "yield"s assuming that the result will be combined
into a string, which means both "f -> str" and "f -> generator" should behave
in the same way.

Before this patch, piping generator function resulted in a cryptic error.
We had to insert "|stringify" in this case.

  $ hg log --template '{if(author, author)|user}\n'
  abort: template filter 'userfilter' is not compatible with keyword
  '[(<function runsymbol at 0x7f5af2e8d8c0>, 'author'),
    (<function runsymbol at 0x7f5af2e8d8c0>, 'author')]'

mercurial/templater.py

@@ -162,8 +162,13 b' def buildfilter(exp, context):'
 
 def runfilter(context, mapping, data):
     func, data, filt = data
+    # func() may return string, generator of strings or arbitrary object such
+    # as date tuple, but filter does not want generator.
+    thing = func(context, mapping, data)
+    if isinstance(thing, types.GeneratorType):
+        thing = stringify(thing)
     try:
-        return filt(func(context, mapping, data))
+        return filt(thing)
     except (ValueError, AttributeError, TypeError):
         if isinstance(data, tuple):
             dt = data[1]

tests/test-command-template.t

@@ -1901,6 +1901,11 b' Thrown an error if a template function d'
   hg: parse error: unknown function 'foo'
   [255]
 
+Pass generator object created by template function to filter
+
+  $ hg log -l 1 --template '{if(author, author)|user}\n'
+  test
+
 Test diff function:
 
   $ hg diff -c 8