templater: make templatepath() not return directory paths

The previous patch added a test showing an unusal error message. This
make it more like other error messages.

Differential Revision: https://phab.mercurial-scm.org/D8804

mercurial/templater.py

@@ -1072,7 +1072,7 b' def templatepath(name):'
     if dir is None:
         return None
     f = os.path.join(templatedir(), name)
-    if f and os.path.exists(f):
+    if f and os.path.isfile(f):
         return f
     return None
 

tests/test-template-map.t

@@ -1284,7 +1284,8 b' Error if style is a directory:'
 Error if style is a directory whose name is a built-in style:
 
   $ hg log --style coal
-  abort: Is a directory: '*/mercurial/templates/coal' (glob)
+  abort: style 'coal' not found
+  (available styles: bisect, changelog, compact, default, phases, show, status, xml)
   [255]
 
 Error if style missing key: