feat: command to simulate Merge/Pull Request

- Adds `git merge(M|P)R <source_branch> <target_branch> [--delete-after-merge]` command
to simulate a Merge/Pull Request being merged into the target branch
- The optional flag `--delete-after-merge` will remove the merged branch on the origin tree
  - The local remote branch remains as it is Git default behavior, even after `git fetch`
(it would require `git fetch --prune` support to automatically prune remote tracking branches)
- You can push a branch with a deleted remote tracking branch on origin, the ref will just update.

Resolves #1057

src/js/git/commands.js

@@ -589,6 +589,47 @@ var commandConfig = {
     }
   },
 
+  mergeMR: {
+    regex: /^git +merge[MP]R($|\s)/,
+    options: ['--delete-after-merge'],
+    execute: function(engine, command) {
+      var generalArgs = command.getGeneralArgs();
+      var commandOptions = command.getOptionsMap();
+      if (!engine.hasOrigin()) {
+        throw new GitError({
+          msg: intl.str('git-error-origin-required'),
+        });
+      }
+
+      command.validateArgBounds(generalArgs, 2, 2);
+
+      var fromBranch = validateOriginBranchName(engine, generalArgs[0]);
+      var intoBranch = validateOriginBranchName(engine, generalArgs[1]);
+
+      var origin = engine.origin;
+
+      origin.checkout(intoBranch);
+      var mergeCommit = origin.merge(fromBranch, { noFF: true });
+
+      origin.animationFactory.genCommitBirthAnimation(
+        origin.animationQueue,
+        mergeCommit,
+        origin.gitVisuals
+      );
+
+      if (!!commandOptions['--delete-after-merge']) {
+        origin.validateAndDeleteBranch(fromBranch);
+      }
+
+      origin.checkout('main');
+
+      origin.animationFactory.playRefreshAnimationAndFinish(
+        origin.gitVisuals,
+        origin.animationQueue
+      );
+    }
+  },
+
   revlist: {
     dontCountForGolf: true,
     displayName: 'rev-list',

src/js/git/index.js

@@ -453,10 +453,17 @@ GitEngine.prototype.findCommonAncestorWithRemote = function(originTarget) {
 };
 
 GitEngine.prototype.makeBranchOnOriginAndTrack = function(branchName, target) {
-  var remoteBranch = this.makeBranch(
-    ORIGIN_PREFIX + branchName,
-    this.getCommitFromRef(target)
-  );
+  var remoteBranch = this.refs[ORIGIN_PREFIX + branchName];
+
+  // If the remote branch exists but the branch on origin was deleted, updates its target location
+  if (remoteBranch) {
+    this.setTargetLocation(remoteBranch, target);
+  } else {
+    remoteBranch = this.makeBranch(
+      ORIGIN_PREFIX + branchName,
+      this.getCommitFromRef(target)
+    );
+  }
 
   if (this.refs[branchName]) { // not all remote branches have tracking ones
     this.setLocalToTrackRemote(this.refs[branchName], remoteBranch);