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/src/repository.ts

@@ -2028,6 +2028,9 @@ export class Repository implements Disposable {
 		// set count badge
 		this.setCountBadge();
 
+		// set mergeChanges context
+		commands.executeCommand('setContext', 'git.mergeChanges', merge.map(item => item.resourceUri.toString()));
+
 		this._onDidChangeStatus.fire();
 
 		this._sourceControl.commitTemplate = await this.getInputTemplate();