loader: Extract cache filename from config object

The only places that this is called, we already have a
reno.config.Config instance available. Simply use that instead.

We also include a note of the 'conf.notespath' property, because the odd
behavior of this caught me out for a bit, and call 'os.path.normpath',
because the default path included redundant up-level references.

Change-Id: I58948cd8fad55d29bd30c65653630bd466259cdc

reno/cache.py

@@ -74,7 +74,7 @@ def write_cache_db(conf, versions_to_include,
         stream = open(outfilename, 'w')
         close_stream = True
     else:
-        outfilename = loader.get_cache_filename(conf.reporoot, conf.notespath)
+        outfilename = loader.get_cache_filename(conf)
         if not os.path.exists(os.path.dirname(outfilename)):
             os.makedirs(os.path.dirname(outfilename))
         stream = open(outfilename, 'w')

reno/config.py

@@ -259,7 +259,13 @@ class Config(object):
 
     @property
     def notespath(self):
-        "The path in the repo where notes are kept."
+        """The path in the repo where notes are kept.
+
+        .. important::
+
+           This does not take ``reporoot`` into account. You need to add this
+           manually if required.
+        """
         return os.path.join(self.relnotesdir, self.notesdir)
 
 # def parse_config_into(parsed_arguments):

reno/loader.py

@@ -21,8 +21,9 @@ from reno import scanner
 LOG = logging.getLogger(__name__)
 
 
-def get_cache_filename(reporoot, notesdir):
-    return os.path.join(reporoot, notesdir, 'reno.cache')
+def get_cache_filename(conf):
+    return os.path.normpath(os.path.join(
+        conf.reporoot, conf.notespath, 'reno.cache'))
 
 
 class Loader(object):
@@ -54,8 +55,7 @@ class Loader(object):
         self._cache = None
         self._scanner = None
         self._scanner_output = None
-        self._cache_filename = get_cache_filename(self._reporoot,
-                                                  self._notespath)
+        self._cache_filename = get_cache_filename(conf)
 
         self._load_data()