formatter: factor out function to create templater from literal or map file

(tmpl, mapfile) will be packed into a named tuple later.

mercurial/cmdutil.py

@@ -1580,15 +1580,8 b' class changeset_templater(changeset_prin'
 
     def __init__(self, ui, repo, matchfn, diffopts, tmpl, mapfile, buffered):
         changeset_printer.__init__(self, ui, repo, matchfn, diffopts, buffered)
-        assert not (tmpl and mapfile)
-        defaulttempl = templatekw.defaulttempl
-        if mapfile:
-            self.t = templater.templater.frommapfile(mapfile,
-                                                     cache=defaulttempl)
-        else:
-            self.t = formatter.maketemplater(ui, 'changeset', tmpl,
-                                             cache=defaulttempl)
-
+        self.t = formatter.loadtemplater(ui, 'changeset', (tmpl, mapfile),
+                                         cache=templatekw.defaulttempl)
         self._counter = itertools.count()
         self.cache = {}
 

mercurial/formatter.py

@@ -408,6 +408,12 b' def lookuptemplate(ui, topic, tmpl):'
 
 def gettemplater(ui, topic, spec, cache=None):
     tmpl, mapfile = lookuptemplate(ui, topic, spec)
+    return loadtemplater(ui, topic, (tmpl, mapfile), cache=cache)
+
+def loadtemplater(ui, topic, spec, cache=None):
+    """Create a templater from either a literal template or loading from
+    a map file"""
+    tmpl, mapfile = spec
     assert not (tmpl and mapfile)
     if mapfile:
         return templater.templater.frommapfile(mapfile, cache=cache)