rebaseline_server: constrain setADir/setBDir to consistently refer to single directories, not lists

gm/rebaseline_server/testdata/outputs/expected/compare_rendered_pictures_test.CompareRenderedPicturesTest.test_repo_url/compare_rendered_pictures.json

 {
   "extraColumnHeaders": {
     "builderA": {
       "headerText": "builderA", 
       "isFilterable": true, 
       "isSortable": true, 
       "useFreeformFilter": false, 
       "valuesAndCounts": [
         [
           null, 
@@ skipping 115 matching lines @@
     "renderModeA", 
     "builderB", 
     "renderModeB"
   ], 
   "header": {
     "dataHash": "-1510211866509185075", 
     "isEditable": false, 
     "isExported": true, 
     "schemaVersion": 5, 
     "setA": {
-      "dir": [
-        "repo:gm/rebaseline_server/testdata/inputs/skp-summaries/expectations"
-      ], 
+      "dir": "repo:gm/rebaseline_server/testdata/inputs/skp-summaries/expectations", 
       "repoRevision": "fake-repo-revision", 
       "section": "expected-results"
     }, 
     "setB": {
-      "dir": [
-        "repo:gm/rebaseline_server/testdata/inputs/skp-summaries/actuals"
-      ], 
+      "dir": "repo:gm/rebaseline_server/testdata/inputs/skp-summaries/actuals", 
       "repoRevision": "fake-repo-revision", 
       "section": "actual-results"
     }, 
     "timeNextUpdateAvailable": null, 
     "timeUpdated": 12345678, 
     "type": "all"
   }, 
   "imagePairs": [
     {
       "extraColumns": {
@@ skipping 68 matching lines @@
     "imageB": {
       "baseUrl": "http://storage.cloud.google.com/fakebucket/fake/path", 
       "description": "actual-results"
     }, 
     "whiteDiffs": {
       "baseUrl": "/static/generated-images/whitediffs", 
       "description": "differing pixels in white"
     }
   }
 }