fix 153492 (#153667)

* enable "accept merge" for merge editor only
* close merge editor before staging file, only stage file when editor has been closed

fixes https://github.com/microsoft/vscode/issues/153492

extensions/git/package.json

@@ -586,7 +586,8 @@
       {
         "command": "git.acceptMerge",
         "title": "%command.git.acceptMerge%",
-        "category": "Git"
+        "category": "Git",
+        "enablement": "isMergeEditor"
       }
     ],
     "keybindings": [