show "Accept Merge" only for files currently under conflict. (#155822)

While the merge editor shows users can handle merge conflicts outside of it, e.g on the console via `git add <FILE>`. The merge editor should have this graceful and step one is to hide the "Accept Merge" command when the file isn't conflicting anymore

* Adds a git-context key that contains all resource-uri-strings under conflict
* Enable/placement of the Accept Merge command is driven by that
* some merge editor context key sugar

extensions/git/package.json

@@ -601,7 +601,7 @@
         "command": "git.acceptMerge",
         "title": "%command.git.acceptMerge%",
         "category": "Git",
-        "enablement": "isMergeEditor"
+        "enablement": "isMergeEditor && mergeEditorResultUri in git.mergeChanges"
       }
     ],
     "keybindings": [
@@ -1549,7 +1549,7 @@
       "merge/toolbar": [
         {
           "command": "git.acceptMerge",
-          "when": "isMergeEditor && baseResourceScheme =~ /^git$|^file$/"
+          "when": "isMergeEditor && mergeEditorBaseUri =~ /^(git|file):/ && mergeEditorResultUri in git.mergeChanges"
         }
       ],
       "scm/change/title": [