stage: hide unwanted warnings (#3763)

These messages all correpond to expected behaviour, so there is no need
to polute the output with unnecessary warnings.

dvc/repo/__init__.py

@@ -1,4 +1,3 @@
-import logging
 import os
 from contextlib import contextmanager
 from functools import wraps
@@ -435,8 +434,7 @@ class Repo(object):
             for file_name in filter(is_valid_filename, files):
                 path = os.path.join(root, file_name)
                 stage_loader = Dvcfile(self, path).stages
-                with stage_loader.log_level(at=logging.DEBUG):
-                    stages.extend(stage_loader.values())
+                stages.extend(stage_loader.values())
                 outs.update(
                     out.fspath
                     for stage in stages

dvc/stage/__init__.py

@@ -246,13 +246,13 @@ class Stage(params.StageParams):
     def stage_changed(self, warn=False):
         changed = self.md5 != self._compute_md5()
         if changed and warn:
-            logger.warning("DVC-file '{}' changed.".format(self.relpath))
+            logger.debug("DVC-file '{}' changed.".format(self.relpath))
         return changed
 
     @rwlocked(read=["deps", "outs"])
     def changed(self):
         if self._changed():
-            logger.warning("{} changed.".format(self))
+            logger.debug("{} changed.".format(self))
             return True
 
         logger.debug("{} didn't change.".format(self))
@@ -831,5 +831,5 @@ class PipelineStage(Stage):
 
     def stage_changed(self, warn=False):
         if self.cmd_changed and warn:
-            logger.warning("'cmd' of {} has changed.".format(self))
+            logger.debug("'cmd' of {} has changed.".format(self))
         return self.cmd_changed

dvc/stage/loader.py

@@ -1,7 +1,6 @@
 import logging
 import os
 from collections import Mapping, defaultdict
-from contextlib import contextmanager
 from copy import deepcopy
 from itertools import chain
 
@@ -30,14 +29,6 @@ class StageLoader(Mapping):
         self.dvcfile = dvcfile
         self.stages_data = stages_data or {}
         self.lockfile_data = lockfile_data or {}
-        self._log_level = logging.WARNING
-
-    @contextmanager
-    def log_level(self, at):
-        """Change log_level temporarily for StageLoader."""
-        self._log_level, level = at, self._log_level
-        yield
-        self._log_level = level
 
     def filter(self, item=None):
         if not item:
@@ -154,11 +145,8 @@ class StageLoader(Mapping):
             raise StageNotFound(self.dvcfile, name)
 
         if not self.lockfile_data.get(name):
-            logger.log(
-                self._log_level,
-                "No lock entry found for '%s:%s'",
-                self.dvcfile.relpath,
-                name,
+            logger.debug(
+                "No lock entry found for '%s:%s'", self.dvcfile.relpath, name,
             )
 
         return self.load_stage(
@@ -187,11 +175,6 @@ class SingleStageLoader(Mapping):
     def filter(self, item=None):
         return self
 
-    @contextmanager
-    def log_level(self, *args, **kwargs):
-        """No-op context manager."""
-        yield
-
     def __getitem__(self, item):
         if item:
             logger.warning(