Remove unnecessary branchName option from the pull command

`git-lfs-pull`'s manpage says "Download all Git LFS files for current ref & checkout", and I don't see any options to pull a specific branch's files.
My guess is that this code was copy-and-pasted from the `fetch` command, explaining the extraneous argument.

Also, this commit has test stubs for the `fetch` and `pull` commands.

Author: Joshua Grosso

src/commands/pull.js

@@ -63,16 +63,12 @@ function pull(repo, options) {
   const args = [];
   const {
     remoteName,
-    branchName,
     callback
   } = (options || {});
 
   if (remoteName) {
     args.push(remoteName);
   }
-  if (branchName) {
-    args.push(branchName);
-  }
   const argsString = R.join(' ', args);
 
   return core.pull(argsString, { cwd: repo.workdir(), shell: true }, callback)

test/tests/commands/fetch.spec.js

@@ -0,0 +1,27 @@
+import {
+  todo
+} from '../../utils';
+
+describe('fetch', () => {
+  describe('the default export', () => {
+    it('runs fetch in the provided repo', todo);
+
+    describe('when a remote name is provided', () => {
+      it('adds the provided remote name to the arguments', todo);
+    });
+
+    describe('when a remote name and a branch name are provided', () => {
+      it('adds the provided remote name and branch name to the arguments', todo);
+    });
+
+    describe('when a callback is provided', () => {
+      it('passes the provided callback to `spawn`', todo);
+    });
+
+    describe('when fetch returns invalid output', () => {
+      it('returns an error response', todo);
+    });
+
+    it('handles errors', todo);
+  });
+});

test/tests/commands/pull.spec.js

@@ -0,0 +1,23 @@
+import {
+  todo
+} from '../../utils';
+
+describe('pull', () => {
+  describe('the default export', () => {
+    it('runs pull in the provided repo', todo);
+
+    describe('when a remote name is provided', () => {
+      it('adds the provided remote name to the arguments', todo);
+    });
+
+    describe('when a callback is provided', () => {
+      it('passes the provided callback to `spawn`', todo);
+    });
+
+    describe('when fetch returns invalid output', () => {
+      it('returns an error response', todo);
+    });
+
+    it('handles errors', todo);
+  });
+});