test-template-engine: do not evaluate unused keywords by custom engine

If the custom engine, "mytemplater", were installed as the default, it would
enter to an infinite recursion at stringify(v) because template keywords may
generate a nested mapping containing the same keywords.

Spotted by a future patch which will replace context.resource('templ')(...)
with context.process(...).

tests/test-template-engine.t

@@ -1,65 +1,64 b''
 
   $ cat > engine.py << EOF
   > 
   > from mercurial import (
   >     pycompat,
   >     templater,
   >     templateutil,
   > )
   > 
   > class mytemplater(templater.engine):
   >     def _load(self, t):
   >         return self._loader(t)
   > 
   >     def process(self, t, map):
   >         tmpl = self._load(t)
   >         props = self._defaults.copy()
   >         props.update(map)
   >         for k, v in props.items():
-  >             if k in (b'templ', b'ctx', b'repo', b'revcache', b'cache',
-  >                      b'troubles'):
+  >             if b'{{%s}}' % k not in tmpl:
   >                 continue
   >             if callable(v) and getattr(v, '_requires', None) is None:
   >                 props = self._resources.copy()
   >                 props.update(map)
   >                 v = v(**pycompat.strkwargs(props))
   >             elif callable(v):
   >                 v = v(self, props)
   >             v = templateutil.stringify(v)
   >             tmpl = tmpl.replace(b'{{%s}}' % k, v)
   >         yield tmpl
   > 
   > templater.engines[b'my'] = mytemplater
   > EOF
   $ hg init test
   $ echo '[extensions]' > test/.hg/hgrc
   $ echo "engine = `pwd`/engine.py" >> test/.hg/hgrc
   $ cd test
   $ cat > mymap << EOF
   > changeset = my:changeset.txt
   > EOF
   $ cat > changeset.txt << EOF
   > {{rev}} {{node}} {{author}}
   > EOF
   $ hg ci -Ama
   adding changeset.txt
   adding mymap
   $ hg log --style=./mymap
   0 97e5f848f0936960273bbf75be6388cd0350a32b test
 
   $ cat > changeset.txt << EOF
   > {{p1rev}} {{p1node}} {{p2rev}} {{p2node}}
   > EOF
   $ hg ci -Ama
   $ hg log --style=./mymap
   0 97e5f848f0936960273bbf75be6388cd0350a32b -1 0000000000000000000000000000000000000000
   -1 0000000000000000000000000000000000000000 -1 0000000000000000000000000000000000000000
 
 invalid engine type:
 
   $ echo 'changeset = unknown:changeset.txt' > unknownenginemap
   $ hg log --style=./unknownenginemap
   abort: invalid template engine: unknown
   [255]
 
   $ cd ..