templater: comment that gettemplate() has different name resolution order

I've tried to unify gettemplate() with buildtemplate(), but it didn't go well
because gettemplate() have to bypass mapping dict.

For example, web templates have '{tags%changelogtag}' and 'changelogtag' is
defined in both mapping, the default, and context.cache, sourced from map file.
In general, mapping shadows context variables, but gettemplate() have to pick
it from context.cache.

mercurial/templater.py

@@ -147,6 +147,9 b' def gettemplate(exp, context):'
     if exp[0] == 'template':
         return compiletemplate(exp[1], context)
     if exp[0] == 'symbol':
+        # unlike runsymbol(), here 'symbol' is always taken as template name
+        # even if it exists in mapping. this allows us to override mapping
+        # by web templates, e.g. 'changelogtag' is redefined in map file.
         return context._load(exp[1])
     raise error.ParseError(_("expected template specifier"))