blink/merge_results.py: Be less verbose

merge_results.py outputs a log line for every file that it 'merges'.
Some of these files are only 'merging' one file. This makes the step log
on buildbot be incredibly verbose, since there are thousands of
artifacts generated by webkit_layout_tests.

This CL changes the behavior to only log something if it's doing a
non-trivial merge. 'Merges' which only merge one file aren't logged.

Change-Id: I6e5bf7316a2f1ab41f8997582b12368c23456378
Reviewed-on: https://chromium-review.googlesource.com/1217594Reviewed-by: Robert Ma <robertma@chromium.org>
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590102}

third_party/blink/tools/blinkpy/web_tests/merge_results.py

@@ -504,7 +504,10 @@ class DirMerger(Merger):
 
             to_merge = [self.filesystem.join(d, partial_file_path) for d in in_dirs]
 
-            _log.debug("Creating merged %s from %s", out_path, to_merge)
+            # If we're only 'merging' one file, don't output to the log. Not a
+            # very useful message.
+            if len(to_merge) > 1:
+              _log.debug("Creating merged %s from %s", out_path, to_merge)
 
             for match_func, merge_func in reversed(self.helpers):